Efficient multi-view coding using depth-map estimate and update

ABSTRACT

The missing of a depth map for a current picture of a reference view—due to the transmission thereof being not anticipated anyway, or due to the advantageous coding order between a texture/picture and its depth map, or due an anticipated discarding of depth data from the bitstream during transmission or decoding—may be adequately addressed so as to reduce inter-view redundancies by estimating a depth map for the pictures of the reference and dependent views and updating same using motion and/or disparity data signaled within the multi-view data stream. In particular, virtually all multi-view data streams have random access points defined therein, i.e. time instances corresponding to pictures of the views of the multi-view signal which are coded without temporal prediction and other dependencies to previously coded pictures, but merely using intra prediction as far as the reference view is concerned, and intra prediction as well as disparity-based prediction as far as the dependent view is concerned. Accordingly, the disparity data signaled within the multi-view data stream for inter-view prediction is exploited to initialize a depth map estimate for the dependent view, and this primary depth map estimate is consecutively updated during the further course of the multi-view coding using motion data and/or disparity data signal within the multi-view data stream. The thus obtained depth map estimate continuously updated, enables the dependent various methods of inter-view redundancy reduction to be performed in a more efficient way than without having access to this depth map estimate. According to another aspect, the following discovery is exploited: the overhead associated with an enlarged list of motion predictor candidates for a block of a picture of a dependent view is comparatively low compared to a gain in motion vector prediction quality resulting from an adding of a motion vector candidate which is determined from an, in disparity-compensated sense, co-located block of a reference view.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Application of U.S. Ser. No.14/272,671, filed May 8, 2014, which is a Continuation Application ofInternational Application No. PCT/EP2012/072299, filed Nov. 9, 2012, andadditionally claims priority from U.S. Application No. 61/558,651, filedNov. 11, 2011. Each of these applications is incorporated herein byreference, in entirety.

The present invention is concerned with multi-view coding in accordancewith a multi-view codec.

BACKGROUND OF THE INVENTION

In multi-view video coding, two or more views of a video scene (whichare simultaneously captured by multiple cameras) are coded in a singlebitstream. The primary goal of multi-view video coding is to provide theend user with an advanced multimedia experience by offering a 3-dviewing impression. If two views are coded, the two reconstructed videosequences can be displayed on a conventional stereo display (withglasses). However, the usage of glasses for conventional stereo displaysis often annoying for the user. Enabling a high-quality stereo viewingimpression without glasses is currently an important topic in researchand development. A promising technique for such autostereoscopicdisplays is based on lenticular lens systems. In principle, an array ofcylindrical lenses is mounted on a conventional display in a way thatmultiple views of a video scene are displayed at the same time. Eachview is displayed in a small cone, so that each eye of the user sees adifferent image; this effect creates the stereo impression withoutspecial glasses. However, such autosteroscopic displays involvetypically 10-30 views of the same video scene (even more views may beused if the technology is improved further). More than 2 views can alsobe used for providing the user with the possibility to interactivelyselect the viewpoint for a video scene. But the coding of multiple viewsof a video scene drastically increases the bit rate that may be used incomparison to conventional single-view (2-d) video. Typically, the bitrate that may be used increases approximately linearly way with thenumber of coded views. A concept for reducing the amount of transmitteddata for autostereoscopic displays consists of transmitting only a smallnumber of views (perhaps 2-5 views), but additionally transmittingso-called depth maps, which represent the depth (distance of the realworld object to the camera) of the image samples for one or more views.Given a small number of coded views with corresponding depth maps,high-quality intermediate views (virtual views that lie between thecoded views)—and to some extend also additional views to one or bothends of the camera array—can be created at the receiver side by asuitable rendering techniques.

For both stereo video coding and general multi-view video coding (withor without depth maps), it is important to exploit the interdependenciesbetween the different views. Since all views represent the same videoscene (from a slightly different perspective), there is a large amountof interdependencies between the multiple views. The goal for designinga highly efficient multi-view video coding system is to efficientlyexploit these interdependencies. In conventional approaches formulti-view video coding, as for example in the multi-view video coding(MVC) extension of ITU-T Rec. H.264|ISO/IEC 14496-10, the only techniquethat exploits view interdependencies is a disparity-compensatedprediction of image samples from already coded views, which isconceptually similar to the motion-compensated prediction that is usedin conventional 2-d video coding. However, typically only a small subsetof image samples is predicted from already coded views, since thetemporal motion-compensated prediction is often more effective (thesimilarity between two temporally successive images is larger than thesimilarity between neighboring views at the same time instant). In orderto further improve the effectiveness of multi-view video coding, it isuseful to combine the efficient motion-compensated prediction withinter-view prediction techniques. One possibility is to re-use themotion data that are coded in one view for predicting the motion data ofother views. Since all views represent the same video scene, the motionin one view is connected to the motion in other views based on thegeometry of the real-world scene, which can be represented by depth mapsand some camera parameters.

In state-of-the-art image and video coding, the pictures or particularsets of sample arrays for the pictures are usually decomposed intoblocks, which are associated with particular coding parameters. Thepictures usually consist of multiple sample arrays (luminance andchrominance). In addition, a picture may also be associated withadditional auxiliary samples arrays, which may, for example, specifytransparency information or depth maps. Each picture or sample array isusually decomposed into blocks. The blocks (or the corresponding blocksof sample arrays) are predicted by either inter-picture prediction orintra-picture prediction. The blocks can have different sizes and can beeither quadratic or rectangular. The partitioning of a picture intoblocks can be either fixed by the syntax, or it can be (at least partly)signaled inside the bitstream. Often syntax elements are transmittedthat signal the subdivision for blocks of predefined sizes. Such syntaxelements may specify whether and how a block is subdivided into smallerblocks and being associated coding parameters, e.g. for the purpose ofprediction. For all samples of a block (or the corresponding blocks ofsample arrays) the decoding of the associated coding parameters isspecified in a certain way. In the example, all samples in a block arepredicted using the same set of prediction parameters, such as referenceindices (identifying a reference picture in the set of already codedpictures), motion parameters (specifying a measure for the movement of ablocks between a reference picture and the current picture), parametersfor specifying the interpolation filter, intra prediction modes, etc.The motion parameters can be represented by displacement vectors with ahorizontal and vertical component or by higher order motion parameterssuch as affine motion parameters consisting of six components. It isalso possible that more than one set of particular prediction parameters(such as reference indices and motion parameters) are associated with asingle block. In that case, for each set of these particular predictionparameters, a single intermediate prediction signal for the block (orthe corresponding blocks of sample arrays) is generated, and the finalprediction signal is built by a combination including superimposing theintermediate prediction signals. The corresponding weighting parametersand potentially also a constant offset (which is added to the weightedsum) can either be fixed for a picture, or a reference picture, or a setof reference pictures, or they can be included in the set of predictionparameters for the corresponding block. The difference between theoriginal blocks (or the corresponding blocks of sample arrays) and theirprediction signals, also referred to as the residual signal, is usuallytransformed and quantized. Often, a two-dimensional transform is appliedto the residual signal (or the corresponding sample arrays for theresidual block). For transform coding, the blocks (or the correspondingblocks of sample arrays), for which a particular set of predictionparameters has been used, can be further split before applying thetransform. The transform blocks can be equal to or smaller than theblocks that are used for prediction. It is also possible that atransform block includes more than one of the blocks that are used forprediction. Different transform blocks can have different sizes and thetransform blocks can represent quadratic or rectangular blocks. Aftertransform, the resulting transform coefficients are quantized andso-called transform coefficient levels are obtained. The transformcoefficient levels as well as the prediction parameters and, if present,the subdivision information is entropy coded.

The state-of-the-art in multi-view video coding extends the 2-d videocoding techniques in a straightforward way. Conceptually, two or morevideo sequences, which correspond to the different views, are coded (ordecoded) in parallel. Or more specifically, for each access unit (ortime instant), the pictures corresponding to the different views arecoded in a given view order. An MVC bitstream contains a base view,which can be decoded without any reference to other views. This ensuresbackwards compatibility with the underlying 2-d video codingstandard/scheme. The bitstream is usually constructed in a way that thesub-bitstream corresponding to the base view (and in additionsub-bitstreams corresponding to particular subsets of the coded views)can be extracted in a simple way by discarding some packets of theentire bitstream. In order to exploit dependencies between views,pictures of already coded views of the current access unit can be usedfor the prediction of blocks of the current view. This prediction isoften referred to as disparity-compensated prediction or inter-viewprediction. It is basically identical to the motion-compensatedprediction in conventional 2-d video coding; the only difference is thatthe reference picture represents a picture of a different view insidethe current access unit (i.e., at the same time instant) and not apicture of the same view at a different time instant. For incorporatinginter-view prediction in the design of the underlying 2-d video codingscheme, for each picture, one or more reference picture lists areconstructed. For the base view (independently decodable view), onlyconventional temporal reference pictures are inserted into the referencepicture lists. However, for all other views, inter-view referencepictures can be inserted into a reference picture list in addition (orinstead of) temporal reference pictures. Which pictures are insertedinto a reference picture list determined by the video codingstandard/scheme and/or signaled inside the bitstream (e.g., in aparameter set and/or slice header). Whether a temporal or inter-viewreference picture is chosen for a particular block of the current viewis then signaled by coding (or inferring) a reference picture index.I.e., the inter-view reference pictures are used in exactly the same wayas conventional temporal reference pictures; only the construction ofthe reference picture lists of slightly extended.

The current state-of-the-art in multi-view video coding is theMulti-view Video Coding (MVC) extension of ITU-T Rec. H.264|ISO/IEC JTC1 [1] [2]. MVC is a straightforward extension of ITU-T Rec.H.264|ISO/IEC JTC 1 towards multi-view video coding. Beside someextensions of the high level syntax, the only tool that has been addedis the disparity-compensated prediction as described above. However, itshould be noted that disparity-compensated prediction is typically onlyused for a small percentage of block. Except for regions that arecovered or uncovered due to the motion inside a scene, the temporalmotion-compensated prediction typically provides a better predictionsignal than the disparity-compensated prediction, in particular if thetemporal distance between the current and the reference picture issmall. The overall coding efficiency could be improved if the temporalmotion-compensated prediction could be combined with suitable inter-viewprediction techniques. There is a conceptually similar problem inscalable video coding, where two representations of the same videosequence with different resolutions or fidelities are coded in a singlebitstream. For the enhancement layer, there are in principle twopossibilities to prediction a block of samples (if we ignore spatialintra prediction), using a temporal motion-compensated prediction froman already coded enhancement layer picture or an inter-layer predictionfrom the lower layer. In Scalable Video Coding (SVC) extension 3, theconventional temporal motion-compensated prediction has been combinedwith an inter-layer prediction of motion parameters. For an enhancementlayer block, it provides the possibility to re-use the motion data ofthe co-located base layer block, but apply it to the enhancement layer(i.e., use the enhancement layer reference picture with base layermotion data). In this way, the temporal motion-compensated predictioninside a layer is efficiently combined with an inter-layer prediction ofmotion data. The general idea behind this technique is that all layersin a scalable bitstream show the same content, and hence also the motioninside each layer is the same. It does not necessarily mean that thebest motion parameters for one layer are also the best motion parametersfor a following layer due to the following effects: (1) The quantizationof the reference pictures modifies the sample values and since differentlayers are quantized differently, the motion parameters that give thesmallest distortion can be different for different layers; (2) Since thelayers are coded at different bit rates, a particular set of motionparameters usually corresponds to a different trade-off between rate anddistortion. And in rate-distortion optimized coding (which is forexample achieved by minimizing of the Lagrangian functional D+λR of thedistortion D and the associated rate R), different motion parameters canbe optimal in rate-distortion sense for different layers (the operatingpoint given by as well as the associated distortion or rate can bedifferent). Nonetheless, the (optimal) motion parameters in base andenhancement layer are usually similar. And it is typically very likelythat a mode the re-uses the motion parameters of the base layer (and istherefore associated with a small rate R) leads to a smaller overallcost (D+λR) than the optimal mode that is independent of the base layer.Or in other words, it is likely that the distortion increase ΔD that isassociated by choosing the mode with base layer motion data instead ofthe mode with optimal enhancement motion data is smaller than the costthat is associated with the decrease in rate (ΔD+λΔR<0).

Conceptually, a similar concept as for SVC can also be used inmulti-view video coding. The multiple cameras capture the same videoscene from different perspective. However, if a real world object movesin the scene, the motion parameters in different captured views are notindependent. But in contrast to scalable coding, where the position ofan object is the same in all layers (a layer represent just a differentresolution or a different quality of the same captured video), theinterrelationship of the projected motion is more complicated anddepends on several camera parameters as well as on the 3-d relationshipsin the real-world scene. But if all relevant camera parameters (such asfocal length, distance of the cameras, and direction of the optical axisof the cameras) as well as the distance of the projected object points(depth map) are given, the motion inside a particular view can bederived based on the motion of another view. In general, for coding avideo sequence or view, we don't need to know the exact motion of theobject points; instead simple parameters such as motion vectors forblocks of samples are sufficient. In this spirit, also the relationshipof the motion parameters between different views can be simplified tosome extent.

However, favorably, the coding order in coding a multi-view signal ischosen such that the pictures conveying the texture of the respectiveview are coded prior to the corresponding depth map so as to be able toefficiently exploit characteristics known from coding/decoding thepicture in coding/decoding the depth map. In even other words, theremoval of redundancy between a depth map and the associated pictureturns out to be more effective in case of a coding order which leadsfrom the picture to the depth map rather than vice versa. Obeying thiscoding order, however, results in a lack of available depth mapinformation at the decoding side at the time the decoder decodes thepicture of a dependent view, since its depth map has not yet beenreconstructed. Disadvantageously, coding parameters of the referenceview may not be exploited efficiently. The situation is even more severein case of multi-view data streams where depth maps of the views do notexist.

SUMMARY

According to an embodiment, an apparatus for reconstructing a multi-viewsignal coded into a multi-view data stream may have a dependent-viewreconstructor configured to derive, for at least one of blocks of acurrent picture in a dependent view of the multi-view signal, a list ofmotion vector predictor candidates, by determining a disparity vectorfor the at least one block, representing a disparity between the currentpicture of the dependent view and a current picture of a reference viewof the multi-view signal at the at least one block of the currentpicture of the dependent view, via motion and disparity vectorsassociated with a previously decoded portion of the multi-view signal;determining a block within the current picture of the reference viewusing the determined disparity vector; adding a motion vector to thelist of motion vector predictor candidates which depends on a motionvector associated with the determined block of the picture of thereference view; and extract, for the at least one block of the currentpicture of the dependent view, index information specifying one motionvector predictor candidate of the list of motion vector predictorcandidates, from the multi-view data stream; and reconstruct the atleast one block of the current picture of the dependent view byperforming a motion-compensated prediction of the at least one block ofthe current picture of the dependent view using a motion vector whichdepends on the specified motion vector candidate.

According to another embodiment, an apparatus for encoding a multi-viewsignal into a multi-view data stream may have a dependent-view encoderconfigured to derive, for at least one of blocks of a current picture ina dependent view of the multi-view signal, a list of motion vectorpredictor candidates, by determining a disparity vector for the at leastone block, representing a disparity between the current picture of thedependent view and a current picture of a reference view of themulti-view signal at the current block of the dependent view, via motionand disparity vectors associated with a previously encoded portion ofthe multi-view signal; determining a block within the current picture ofthe reference view using the determined disparity vector; adding amotion vector to the list of motion vector predictor candidates whichdepends on a motion vector associated with the determined block of thepicture of the reference view; and insert, for the at least one block ofthe current picture of the dependent view, index information specifyingone motion vector predictor candidate of the list of motion vectorpredictor candidates, into the multi-view data stream; and encode the atleast one block of the current picture of the dependent view byperforming a motion-compensated prediction of the at least one block ofthe current picture of the dependent view using a motion vector whichdepends on the specified motion vector candidate.

According to another embodiment, an apparatus for reconstructing amulti-view signal coded into a multi-view data stream may have: areference-view reconstructor configured to reconstruct a current pictureof a reference view of the multi-view signal using motion compensatedprediction based on motion data transmitted within the multi-view datastream for the reference view; and a depth estimator configured toestimate the depth map of the current picture of the dependent view bygenerating a depth map estimate of the current picture of the referenceview by applying the motion data for the reference view onto a depth mapestimate of a previous picture of the reference view; and warping thedepth map estimate of the current picture of the reference view into thedependent view so as to acquire the depth map estimate of the currentpicture of the dependent view; a dependent-view reconstructor configuredto reconstruct the current picture of the dependent view from adependent view portion of the multi-view data stream using the depth mapestimate of the current picture of the dependent view.

According to another embodiment, an apparatus for encoding a multi-viewsignal into a multi-view data stream may have: a reference-view encoderconfigured to encode a current picture of a reference view of themulti-view signal using motion compensated prediction based on motiondata for the reference view with transmitting the motion data for thereference view via the multi-view data stream; and a depth estimatorconfigured to estimate the depth map of the current picture of thedependent view by generating a depth map estimate of the current pictureof the reference view by applying the motion data for the reference viewonto a depth map estimate of a previous picture of the reference view;and warping the depth map estimate of the current picture of thereference view into the dependent view so as to acquire the depth mapestimate of the current picture of the dependent view; a dependent-viewencoder configured to encode the current picture of the dependent viewinto a dependent view portion of the multi-view data stream using thedepth map estimate of the current picture of the dependent view.

According to another embodiment, a method for reconstructing amulti-view signal coded into a multi-view data stream may have the stepsof: deriving, for at least one of blocks of a current picture in adependent view of the multi-view signal, a list of motion vectorpredictor candidates, by determining a disparity vector for the at leastone block, representing a disparity between the current picture of thedependent view and a current picture of a reference view of themulti-view signal at the at least one block of the block of the currentpicture of the dependent view, via motion and disparity vectorsassociated with a previously decoded portion of the multi-view signal;determining a block within the current picture of the reference viewusing the determined disparity vector; adding a motion vector to thelist of motion vector predictor candidates which depends on a motionvector associated with the determined block of the picture of thereference view; and extracting, for the at least one block of thecurrent picture of the dependent view, index information specifying onemotion vector predictor candidate of the list of motion vector predictorcandidates, from the multi-view data stream; and reconstructing the atleast one block of the current picture of the dependent view byperforming a motion-compensated prediction of the at least one block ofthe current picture of the dependent view using a motion vector whichdepends on the specified motion vector candidate.

According to another embodiment, a method for encoding a multi-viewsignal into a multi-view data stream may have the steps of: deriving,for at least one of blocks of a current picture in a dependent view ofthe multi-view signal, a list of motion vector predictor candidates, bydetermining a disparity vector for the at least one block, representinga disparity between the current picture of the dependent view and acurrent picture of a reference view of the multi-view signal at thecurrent block of the dependent view, via motion and disparity vectorsassociated with a previously encoded portion of the multi-view signal;determining a block within the current picture of the reference viewusing the determined disparity vector; adding a motion vector to thelist of motion vector predictor candidates which depends on a motionvector associated with the determined block of the picture of thereference view; and inserting, for the at least one block of the currentpicture of the dependent view, index information specifying one motionvector predictor candidate of the list of motion vector predictorcandidates, into the multi-view data stream; and encoding the at leastone block of the current picture of the dependent view by performing amotion-compensated prediction of the at least one block of the currentpicture of the dependent view using a motion vector which depends on thespecified motion vector candidate.

According to another embodiment, a method for reconstructing amulti-view signal coded into a multi-view data stream may have the stepsof: reconstructing a current picture of a reference view of themulti-view signal using motion compensated prediction based on motiondata transmitted within the multi-view data stream for the referenceview; and estimating the depth map of the current picture of thedependent view by generating a depth map estimate of the current pictureof the reference view by applying the motion data for the reference viewonto a depth map estimate of a previous picture of the reference view;and warping the depth map estimate of the current picture of thereference view into the dependent view so as to acquire the depth mapestimate of the current picture of the dependent view; reconstructingthe current picture of the dependent view from a dependent view portionof the multi-view data stream using the depth map estimate of thecurrent picture of the dependent view.

According to another embodiment, a method for encoding a multi-viewsignal into a multi-view data stream may have the steps of: encoding acurrent picture of a reference view of the multi-view signal usingmotion compensated prediction based on motion data for the referenceview with transmitting the motion data for the reference view via themulti-view data stream; and estimating the depth map of the currentpicture of the dependent view by generating a depth map estimate of thecurrent picture of the reference view by applying the motion data forthe reference view onto a depth map estimate of a previous picture ofthe reference view; and warping the depth map estimate of the currentpicture of the reference view into the dependent view so as to acquirethe depth map estimate of the current picture of the dependent view;encoding the current picture of the dependent view into a dependent viewportion of the multi-view data stream using the depth map estimate ofthe current picture of the dependent view.

According to another embodiment, a computer program may have a programcode for performing, when running on a computer, a method according toclaim 18.

According to another embodiment, a computer program may have a programcode for performing, when running on a computer, a method according toclaim 20.

In accordance with a first aspect of the present invention, an idea isexploited according to which the missing of a depth map for a currentpicture of a reference view—due to the transmission thereof being notanticipated anyway, or due to the advantageous coding order between atexture/picture and its depth map, or due an anticipated discarding ofdepth data from the bitstream during transmission or decoding—may beadequately addressed so as to reduce inter-view redundancies byestimating a depth map for the pictures of the reference and dependentviews and updating same using motion and/or disparity data signaledwithin the multi-view data stream. In particular, virtually allmulti-view data streams have random access points defined therein, i.e.time instances corresponding to pictures of the views of the multi-viewsignal which are coded without temporal prediction and otherdependencies to previously coded pictures, but merely using intraprediction as far as the reference view is concerned, and intraprediction as well as disparity-based prediction as far as the dependentview is concerned. Accordingly, the disparity data signaled within themulti-view data stream for inter-view prediction may be exploited toinitialize a depth map estimate for the dependent view, and this primarydepth map estimate may be consecutively updated during the furthercourse of the multi-view coding using motion data and/or disparity datasignal within the multi-view data stream. The thus obtained depth mapestimate continuously updated, enables the dependent various methods ofinter-view redundancy reduction to be performed in a more efficient waythan without having access to this depth map estimate.

According to another aspect, the following discovery is exploited: theoverhead associated with an enlarged list of motion predictor candidatesfor a block of a picture of a dependent view is comparatively lowcompared to a gain in motion vector prediction quality resulting from anadding of a motion vector candidate which is determined from an, indisparity-compensated sense, co-located block of a reference view. Thedisparity between both blocks may or may not be determined using thefirst aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be detailed subsequentlyreferring to the appended drawings, in which:

FIG. 1 shows a block diagram of an apparatus for reconstruction of amulti-view signal in accordance with an embodiment;

FIG. 2 shows a block diagram of an apparatus for encoding a multi-viewsignal fitting to the apparatus of FIG. 1 in accordance with anembodiment;

FIG. 3 shows a general relationship between projected objects points,temporal motion vectors, and disparity vectors in the pictures ofdifferent views and time instances;

FIG. 4 shows a basic process for deriving a motion vector for thecurrent block given the motion in a reference view and a depth mapestimate for the current picture (using a particular sample positioninside the current block);

FIG. 5 shows a basic process for mapping a depth map given for one viewto another view: (left) given depth map for a view, where the grey arearepresents a background and white area represents a foreground object;(middle) converted depth map obtained by displacing the samples with thedisparity vectors that corresponds to the depth values and keeping theforeground object for locations to which more than one sample isprojected, the black area represents on disoccluded area to which nosample has been projected; (right) converted depth map after filling thedisoccluded areas by the depth value for the background;

FIG. 6 shows a generation of depth maps (using disparity vectors) for arandom access unit;

FIG. 7 shows temporal prediction of an estimated depth map using themotion parameters coded in the base view; and

FIG. 8 shows an update of the depth map using actually coded motion anddisparity vectors.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an embodiment for an apparatus 10 for reconstructing amulti-view signal 12 coded into a multi-view data stream 14. Theapparatus 10 comprises an input 16 for the multi-view data stream 14,and two outputs 18 a and 18 b for a reference view signal 20 and adependent view signal 22, respectively.

Further, apparatus 10 comprises a reference view reconstructor 24connected between input 16 and output 18 a and a dependent viewreconstructor 26 connected between input 16 and output 18 b. A depth mapestimator 28 of apparatus 10 is connected between a parameter output ofreference view reconstructor 24 and a reference input of dependent viewreconstructor 26, and to a parameter output of dependent viewreconstructor 26.

As will be outlined in more detail below, the apparatus or decoder 10 ofFIG. 1 reconstructs the multi-view signal 12 from the multi-view datastream 14 by obeying a coding/decoding order according to which thereference signal 20 is processed prior to dependent view 22. Themulti-view signal 12 may, as shown in FIG. 1, not only represent aspatial sampling of one common scene from different view directions orview points associated with respective views 20 and 22, but also atemporal sampling of this scene as it is illustrated in FIG. 1exemplarily by showing three time instants T−1, T and T+1 along a timeaxis 30. For each time instant, each view 20 and 22 comprises a picture32 t ₁ and 32 t ₂, wherein each picture 32 t _(1,2) represents arespective texture map.

It is noted that FIG. 1 assumes that both views 20 and 21 have theirpictures 32 t _(1,2) temporally aligned. However, the time resolutionbetween view 20 and view 22 may differ. Naturally, the same applies tothe spatial resolution of the pictures and depth maps.

Moreover, decoder 10 is configured to process the multi-view signal 12sequentially in time. To be more precise, decoder 10 is configured toreconstruct the pictures 32 t _(1,2) of a certain time instance, such asT−1, prior to continuing with processing the pictures and depth maps ofthe subsequent time instance T. In this regard, it is noted that thetemporal coding order among the time instances of the multi-view signal12 may be equal to the presentation time order of the pictures and depthmaps, respectively, or may differ therefrom.

The reference view reconstructor 24 is configured to reconstruct thereference view 20 from a reference view portion 36 of the multi-viewdata stream 14, while the dependent view reconstructor 26 is configuredto reconstruct the dependent view 22 based on a dependent view portion38 of the multi-view data stream 14. In fact, reference viewreconstructor 24 and dependent view reconstructor 26 may be configuredto operate in a similar manner. For example, reference reconstructor 24and dependent view reconstructor 26 may operate on a block-wise basis.Both may, for example, be configured as a hybrid video decoder,respectively. The reference view reconstructor 24 reconstructs, forexample, the picture 32 t ₁ of a current time instant T by assigning arespective one of available coding modes to the blocks 40 into whichthis picture is subdivided. The subdivision of picture 32 t ₁ intoblocks may be predefined by default or may be signaled within themulti-view data stream 14. The subdivision may subdivide picture 32 t ₁in a regular manner into blocks of the same size or blocks of differentsize. Even further, a multi-tree subdivisioning may be possible so thatthe block size of blocks 40 may be locally adapted to the picturecontent. The coding modes available may comprise one or more intraprediction modes according to which reference view reconstructor 24fills the respective block 40 by prediction from already reconstructedsamples of already reconstructed blocks preceding the current block in adecoding order defined among the blocks of picture 32 t ₁, one or moreinter prediction modes according to which reference view reconstructor24 reconstructs the respective block by motion compensated and/orprediction using motion data such as motion vectors, reference pictureindices and the like. For example, for illustration purposes two blocksare exemplarily shown to be reconstructed by inter prediction. Themotion data 42 for these inter-predicted blocks may comprise motionvectors used by reference view reconstructor 24 to copy respectiveportions of a reconstructed version of a reference picture 32 t ₁indexed by a motion reference index also comprised by the motion data42. The motion data 42 is comprised by the reference view portion 36 ofmulti-view data stream 14.

The dependent view reconstructor 26 operates quite the same as referenceview reconstructor 24 with dependent view reconstructor 26, however,being configured to reconstruct the dependent view 22 from the dependentview portion 38. Accordingly, in reconstructing a current picture 32 t ₂of current time instant T, dependent view reconstructor 26 may also usea block-wise processing using a subdivision into blocks 50 which may befixed or signaled within multi-view data stream 14. Alternatively, depthmap based inter-view prediction of the subdivision into blocks 50 asoutlined in more detail below may be used by dependent viewreconstructor 26 so as to derive the subdivision into blocks 50 for view22 from the subdivision into blocks 40 of view 20. As far as the codingmodes are concerned, dependent view reconstructor 26 may support codingmodes as they have been described with respect to the reference viewreconstructor 24. Accordingly, illustratively, two blocks 50 areexemplarily shown to be subject to inter prediction using motion data54, respectively, so as to be appropriately copied from respectiveportions of a reconstructed version of previously reconstructed pictures32 t ₂, respectively. Together, this motion data 58 represents themotion data for the current picture or current time instance of view 22.In addition to these coding modes, however, dependent view reconstructor26 has the ability to support one or more inter-view prediction modesfor using disparity-compensated prediction in order to copy respectiveblocks from portions of view 20 of the same time instance, but spatiallydisplaced, as defined by some disparity data. In FIG. 1, one disparitypredicted block in picture 32 t ₂ is exemplarily shown along with thecorresponding disparity data 60. Disparity data 60 may, for example,comprise a disparity vector or at least a disparity component along theview offset direction between views 20 and 22, and optionally a viewindex indicating the reference view from which the respective block 50of the dependent view 22 depends, which index may be favorable m case ofthe coexistence of more than two views as exemplarily shown in FIG. 1.

That is, reference view reconstructor 24 and dependent viewreconstructor 26 operate in a manner so as to reduce the redundanciesalong the time axis 30 and in inter-view direction, between views 20 and22, as far as possible. This is also true, for example, for theprediction of the side information such as the motion data and disparitydata as well as the coding modes and the subdivision informationmentioned above. All of this information shows redundancies among eachother in time direction, and between the views.

However, the dependent view reconstructor 26 could more efficientlyexploit the redundancy between views 20 and 22 if the dependent viewreconstructor 26 had access to a depth map for a currently decodedpicture 32 t ₂. Accordingly, the depth estimator 28 is configured toprovide a depth map estimate 64 as an estimate for a depth map of thecurrent picture 32 t ₂ of the current time instant T in the mannerdescribed in more detail below, and the dependent view reconstructor 26is configured to reconstruct the current picture 32 t ₂ of the currenttime instant of the dependent view 22 from the dependent view portion 38of the multi-view data stream 14 using this depth map estimate 64. Forexample, having the depth map estimate 64 at hand, the dependent viewreconstructor 26 is able to predict the motion data 54 of the currentpicture of the dependent view 22 based on the depth map estimate 64 ofthe current view 22 and the motion data 42 for the current picture ofthe reference view 20 and reconstruct the current picture of thedependent view 22 using motion compensated prediction based on thepredicted motion data. For example, the current-view reconstructor 24may be configured to, in predicting the motion data 54, use the depthdata estimate 64 of the dependent view 22 to locate correspondingpositions in the current picture of the reference view 20 and use themotion data 42 for the current picture of the reference view 20 at thecorresponding positions to predict the motion data 54 of the currentpicture of the dependent view 22. In the following description, apossible way how the spatial look-up using the depth data estimate 64 isdone is described in more detail below. In particular, in the followingdescription, the fact that the motion data 42 forms a good predictor forthe motion data 54 is motivated in more detail. Naturally, refinementdata in order to signal a prediction residual for the motion data 54 maybe comprised by the dependent view portion 38. In particular, as will beset out in more detail below, dependent view reconstructor 26 may beconfigured to apply disparity vectors derived from the depth dataestimate 64 for one or more predetermined sample positions within acurrent block 50 of picture 32 t ₂ of the current time instant ofdependent view 22 and use these disparity vectors in order to locatecorresponding or warped positions in picture 32 t ₁ of the same timeinstant of view 20 with using the motion data 42 of the one or moreblocks 40 containing the one or more warped positions as a predictor forthe motion data 54 of the current block 50. In case of more than onesuch reference sample position within the current block 50, the mean ormedian value of the resulting one or more reference motion data of thetargeted block or blocks 40 may be used as the predictor.

Further, the dependent view reconstructor 26 could be configured topredict the disparity data 60 for the current picture of the dependentview 22 based on the depth data estimate 64 of the dependent view 22 andreconstruct the current picture of the dependent view 22 using disparitycompensated prediction based on the predicted current disparity data.Again, refinement may be signaled within dependent view portion 38 ofthe multi-view data stream 14 and used by dependent view reconstructor26 to refine the predicted current disparity data. Further, as outlinedabove, theoretically the disparity data 60 of blocks 50 could bepredicted too in the same way. As will be outlined in more detail below,the dependent view reconstructor 26 could be configured to predict thedisparity data 60 and 62 based on the depth data estimate 64 of thecurrent view by converting the depth data into disparity vectors andusing these disparity vectors as a predictor for the disparity vectorswithin the disparity data 60 and 62, respectively, directly.

Naturally, dependent view reconstructor 26 could support any combinationof the just-mentioned possibilities so as to use the depth data estimateso as to reduce the inter-view redundancy between views 20 and 22.

In order to derive the just-mentioned depth data estimate 64, the depthestimator 28 acts as follows.

In particular, in accordance with the embodiment of FIG. 1, the depthestimator 28 ensures that each picture 32 t _(1,2) has an depth mapestimate 64 associated therewith which are consecutively derived fromeach other in a chain of updates. As will be outlined in more detailbelow, depth estimator 28 is configured to continuously update the depthmap estimates 64 in a ping pong manner between views 20 and 22 primarilywith the aim to provide each picture 32 t ₂ of dependent view 22 withsuch a depth map estimate 64 in order to serve as a basis for theabove-outlined improved inter-view redundancy reduction.

Preliminarily, it is assumed that depth estimator 28 already has accessto such a depth estimate for one or more previous pictures 32 t ₁ of thereference view 20 such as time instance T−1. A way how depth estimator28 could have gained access to this depth map estimate 74 for thepreviously decoded picture 32 t ₁ of the reference view 20 is describedfurther below. It should be noted, however, that such depth map datacould be intermittently signaled explicitly within the multi-view datastream 14 for first pictures 32 t ₁ of the reference view 20 within socalled random access units, i.e. groups of pictures 32 t ₁ which aredecodable without reference to any previous portions of signal 12. Inorder to illustrate this possibility, a dashed line connects depthestimator 28 with input 16. In the following description, a possibilityis presented where the extra transmission of such starting depth map isnot necessary. Rather, the disparity data within the data stream portion38 for the first picture 32 t ₂ of the dependent view 22 in coding orderwithin the random access unit is exploited to construct the startingdepth map of the first picture 32 t ₁ of the reference view 20 in codingorder within the random access unit.

In particular, the depth estimator 28 is configured to generate thedepth map 64 of the current picture 32 t ₂ of the dependent view 22 byapplying the motion data 42 for the current picture 32 t ₁ of thereference view 20 at the current time instance T onto the depth mapestimate 74 of any previous picture 32 t ₁ of the reference view 20 atthe time instant T−1, for example. As already noted above, thereference-view reconstructor 24 reconstructs the current picture 32 t ₁of the reference view 20 using motion compensated prediction based onthe motion data 42, which is signaled within the multi-view data stream14 for the reference view 20. The depth estimator 28 has access to thismotion data 42 and uses this motion data 42 for one of the mentionedupdates of the chain of updates, namely the transition 71 from the depthmap estimate 74 of the reference picture 32 t ₁ at the previous timeinstant T−1 to the depth map estimate 64 of the current picture 32 t ₁at the current time instant T. A way how this may be performed will beoutlined in more detail below. Preliminarily, it shall be sufficient tonote that applying 71 the motion data 42 onto the depth map 74 for theprevious time instance T−1 could mean that co-located blocks 72, i.e.portions within depth map estimate 64 of the current picture 32 t ₁which are co-located to blocks 40 for which this motion data 42 has beensignaled in the stream portion 36, are updated with, i.e. copied from,content of the referenced depth map estimate, i.e. the depth mapestimate 74 for the picture 32 t ₁ of the previous time instance T−1 atportions within the referenced depth map estimate 74 pointed to by themotion data 42′ equal to motion data 42. Remaining holes may be filledby interpolation and/or extrapolation exploiting additional informationoffered by the intra-coded blocks among block 40 of the current picture32 t ₁. As a result, the depth map estimate 64 has been updated (orgenerated by transitioning from T−1 to T).

Again, depth estimator 28 performs this update/transition 71 merely inorder to prosecute further the chain of updates described further belowso as to serve as a basis for deriving the depth map estimate 64 of thecurrent picture 32 t ₂ of the dependent view 22 of the same timeinstants T. To finalize the derivation, depth estimator 28 warps theupdated depth map estimate 64 of the current picture 32 t ₁ of thereference view 20 into the dependent view 22 so as to obtain the depthmap estimate 64 of the current picture 32 t ₂ of the dependent view 22.That is, as the motion data 42 is defined merely at a block granularity,the update/transition 71 and the resulting depth map estimate 64 of view22 as resulting from the warping 78 represent a quite coarse estimationof the depth, but as will be shown below such a coarse estimate issufficient in order to significantly increase the efficiency inperforming the inter-view redundancy reduction.

Although possible details regarding the warping 76 are also describedfurther below, briefly spoken, the dependent-view reconstructor 26 maybe configured to perform the warping 78 by deriving disparity vectorsfrom the depth map estimate 64 of current picture 32 t ₁ and applyingthe derived disparity vectors onto the depth map estimate 64 itself, soas to obtain the warped depth map estimate 64 of the current picture 32t ₂ of the dependent view 22.

Thus, as soon as depth estimator 28 has provided dependent-viewreconstructor 26 with the result of the warping 76, namely the depth mapestimate 64 of the current time instant T for view 22, dependent viewreconstructor 26 is able to use this depth map estimate 64 forperforming the above-outlined inter-view redundancy reduction for whichpossible implementations are set out in more detail below.

However, depth estimator 28 continues to update 77 this depth mapestimate 64 so as to obtain an updated depth map estimate 74 for thecurrent picture 32 t ₂ of the reference view 22 and thereby maintainingthe chain of updates leading to the estimate for the next time instanceT+1. Accordingly, the dependent-view reconstructor 26 is configured toupdate 77 the depth map estimate 64 of the current picture 32 t ₂ of thedependent view 22 of the current time instance T using the disparityand/or motion data 54 and 60 for the dependent view 22 in a mannersimilar, at least for the motion data 54, as described above withrespect to the update step 71. That is, the dependent view reconstructor26 uses the disparity/motion data for the picture 32 t ₂ for timeinstance T within stream portion 38 for reconstructing this picture 32 t₂. As far as the disparity data 60 is concerned, depth estimator 28 mayeasily convert the disparity vectors contained within the disparity data54 into depth values and assign, based on these depth values, updateddepth values to samples of the updated depth map estimate 79 b of thecurrent picture 32 t ₂ of the dependent view 22 which are co-located tothe respective disparity-predicted block 50 in picture 32 t ₂. Themotion data 54 could be used so as to copy content of the depth mapestimate 74 of the picture 32 t ₂ of a referenced previous time instanceT−1 of the dependent view 22, at portions thereof pointed to by motiondata equal to motion data 54, into portions within the updated depth mapestimate 74 of the current picture 32 t ₂ which are co-located to blocks50 for which this motion data 42 has been signaled in the stream portion36. Remaining holes may be filled by interpolation and/or extrapolationexploiting additional information offered by the intra-coded blocksamong block 40 of the current picture 32 t ₁. As a result, the updateddepth map estimate 74 of the current picture 32 t ₂ has been updated (orgenerated by transitioning from T−1 to T). A possibility for as to howthe depth map estimate 74 of the picture 32 t ₂ of a referenced previoustime instance T−1 of the dependent view 22 may have been derived at thebeginning of an random access unit, is described further below. However,the above mentioned possibly explicitly transmitted depth map for view20 at the beginning of such random access unit may be warped to view 22to obtain the depth map estimate 74 of the picture 32 t ₂ of areferenced previous time instance T−1 of the dependent view 22,alternatively.

In order to reduce blocking artifacts, the updates 71 and 77 could beperformed by using weighting functions reducing the influence of theupdates of the individual blocks at the block borders.

That is, on the basis of the depth map estimate 64 as obtained bywarping 76, the dependent-view reconstructor 26 reconstructs the currentpicture 32 t ₂ of dependent view 22 using disparity and/or motioncompensated prediction based on the disparity and/or motion data 54 and60 for the dependent view 22 comprised by the dependent view portion 38of the multi-view data stream 14, and in doing so, the dependent-viewreconstructor 26 provides the depth estimator 28 with the disparityand/or motion data 54, 60, then used by depth estimator 68 to performupdate 77.

After this update 77, the depth estimator 28 is able to warp-back 78 theupdated depth map estimate 74 of the current picture 32 t ₂ of thedependent view 22 into the reference view 20 so as to obtain the updateddepth map estimate 74 of current picture 32 t ₁ of the reference view 20for a time instance T which may then serve as a basis/reference for thetransition/update 79 to the next time instance T+1 and so forth.

From that time on, depth estimator 28 merely repeats processes 71, 76,77 and 78 iteratively (wherein step 79 corresponds to step 71) so as tomodel the depth map estimate along the time axis 30 so as tocontinuously support the dependent view reconstructor 26 with the depthmap estimate 64.

Further details regarding all these steps 71, 76, 77, 78, and 79 aredescribed in further detail below. All of these further details shall beindividually applicable to the description brought forward with regardto FIG. 1.

Before describing further details regarding the concepts outlined above,an embodiment for an encoder fitting to the decoder of FIG. 1 isdescribed with respect to FIG. 2. FIG. 2 shows an apparatus for encodingthe multi-view signal 12 into the multi-view data stream 14 andcomprises, to this end, a reference view encoder 80, a dependent viewencoder 82 and a depth estimator 84 with the encoder generally indicatedwith reference sign 90. The reference view encoder 80 is configured toencode the reference view 20 of the multi-view signal 12 into thereference view portion 36 of the data stream 14, while dependent viewencoder 82 is responsible for encoding the dependent view 22 ofmulti-view signal 12 into the dependent view portion 38 of themulti-view data stream 14. Reference view encoder 80 and dependent viewencoder 82 may operate in a backward predictive manner and the depthestimator 84 may be configured to perform the depth map estimate and itscontinuous update in the manner described above with respect to thedecoder 10 by using the same information available from reference viewencoder 80 and dependent-view encoder 82. That is, the depth estimator84 is configured to generate 71 a depth map estimate 64 of the currentpicture 32 t ₂ of the dependent view 22 by applying motion data 42 forthe reference view having been used to motion compensatedly predict thecurrent picture of the reference view, onto a depth map estimate of aprevious picture 32 t ₁ of the reference view 20 and warping 76 the thusobtained depth map estimate 64 of the current picture 32 t ₁ of thereference view 20 into the dependent view 22 so as to obtain the depthmap estimate 64 of the current picture 32 t ₂ of the dependent view 22.Likewise, depth estimator 84 also performs the subsequent update step 77and the following back-warp step 78. To this end, reference view encoder80 and dependent view encoder 82 may be parallely connected between aninput and an output of encoder 90, while depth estimator 84 may beconnected between a parameter output of reference view encoder 80 and areference input of dependent view encoder 82 and connected to aparameter output of dependent view encoder 82. The reconstruction outputof reference view encoder 80 may be connected to an prediction parameteroutput of reference view encoder 80 such as an output of an internalprediction block.

The dependent-view encoder 82 may encode the current picture or currenttime instant of the dependent view 22 using the depth map estimate 64 inthe manner outlined above with respect to FIG. 1, namely for predictingmotion data 58 or at least 54, or predicting disparity data 60 and 62 orat least 60, or at least a part of these options, and with or withoutgenerating prediction residual data for the respective motion ordisparity data, so as to form a part of the dependent view portion 38.

In the following, more detailed embodiments are presented, which areespecially advantageous when combined with hybrid coding types usingblock merging, multi-tree block partitioning of regularly arrangedtree-root blocks such as in HEVC.

The state-of-the art concepts for employing motion data of a referenceview for efficiently coding a further view have all been developed basedon the MVC extension of ITU-T Rec. H.264|ISO/IEC 14496-10. The new videocoding standardization project of the ITU-T and ISO/IEC JTC 1/WG 11,which is also referred to as HEVC, shows very promising improvements inconventional 2-d video coding technology. The current working draft ofHEVC provides substantial coding gains compared to ITU-T Rec.H.264|ISO/IEC 14496-10. For achieving these gains several concepts havebeen extended in comparison to ITU-T Rec. H.264|ISO/IEC 14496-10. Themain improvements in the area of motion parameter coding andmotion-compensated prediction include the following:

-   -   While the blocks sizes that are used for motion-compensated        prediction in ITU-T Rec. H.264|ISO/IEC 14496-10 range from 4×4        to 16×16 luma samples, a much larger variety of blocks sizes is        supported in HEVC, which ranges from 4×4 to 64×64 luma samples.        In addition, the basic coding units are not given by fixed        macroblock and sub-macroblocks, but are adaptively chosen. The        largest coding unit is typically a block of 64×64 luma samples,        but the largest block size can actually be signaled inside the        bitstream. The splitting of a block into subblock can establish        a subdivision hierarchy of 4 or more levels.    -   Motion vectors are not coded by using a fixed motion vector        predictor. Instead there exists a list of motion vector        predictor candidates, and one of these predictors is adaptively        chosen on a block basis. The chosen predictor is signaled inside        the bitstream.    -   ITU-T Rec. H.264|ISO/IEC 14496-10 provides the SKIP and DIRECT        for which the motion parameters (number of hypothesis, reference        indices, motion vectors) are completely derived from already        coded information, without coding any additional parameters        (except residual information). HEVC provides a so-called merge        mode. For this mode a list of motion parameter candidates given        by the motion parameters of spatially and temporally neighboring        blocks is established. The motion parameters (including the        number of hypothesis, reference indices, and motion vectors)        that are chosen for a block coded in the merge mode are signaled        by transmitting an index into the candidate list.

The following description will describe a concept for employing themotion data of already coded views as well as the disparity data foralready coded pictures of a current view for coding a current picture ofthe current view in multiview video coding, with this conceptrepresenting a possible implementation of the embodiment describedabove. Further, the advantages resulting from the above and followingembodiments will be explained in more detail. By employing the alreadycoded motion and disparity information for predicting the temporalmotion (or the disparity) of the current view, the motion data rate forthe current view can be significantly reduced, which results in anoverall bit rate saving for the coding of multiview video sequences. Thedescribed concept provides the possibility to directly derive all motiondata for a block (or a general set of samples), in which case no furthermotion information are transmitted for a block. And it also provides thepossibility to derive a motion vector predictor that is added to a listof candidate motion vector predictors. For the latter possibility, anindex into the list of motion vector predictors as well as a motionvector difference are transmitted for a block, which specify the finalmotion vector used for motion-compensated prediction. In a particularembodiment of the invention, not only the motion parameters for a block,but also the partitioning information for the block (which can split theblock into smaller blocks and assign separate motion parameters to eachsub-block) can be derived based on the already coded motion anddisparity information. The concept is applicable to general block-basedhybrid coding approaches without assuming any particular macroblock orsub-macroblock structure. The general block-based motion compensation isnot modified, but only the coding of motion parameters, so that theconcept can be integrated in general block-based hybrid video codingschemes with a very small complexity increase. It can also bestraightforwardly extended to more general concepts, in which notrectangular blocks but other sets of samples are associated for uniquemotion parameters. The concept is applicable to multiview coding withand without additional depth maps. The disparity information forcalculating the motion parameters can be derived based on coded depthmaps based on coded disparity vectors.

The following description will describe a concept for employing themotion data of already coded views as well as the disparity data foralready coded pictures of a current view for coding a current picture ofthe current view in multiview video coding, with this conceptrepresenting a possible implementation of the embodiment describedabove. Further, the advantages resulting from the above and followingembodiments will be explained in more detail. By employing the alreadycoded motion and disparity information for predicting the temporalmotion (or the disparity) of the current view, the motion data rate forthe current view can be significantly reduced, which results in anoverall bit rate saving for the coding of multiview video sequences. Theinvention provides the possibility to directly derive all motion datafor a block (or a general set of samples), in which case no furthermotion information are transmitted for a block. And it also provides thepossibility to derive a motion vector predictor that is added to a listof candidate motion vector predictors. For the latter possibility, anindex into the list of motion vector predictors as well as a motionvector difference are transmitted for a block, which specify the finalmotion vector used for motion-compensated prediction. In a particularembodiment of the invention, not only the motion parameters for a block,but also the partitioning information for the block (which can split theblock into smaller blocks and assign separate motion parameters to eachsub-block) can be derived based on the already coded motion anddisparity information. The concept is applicable to general block-basedhybrid coding approaches without assuming any particular macroblock orsub-macroblock structure. The general block-based motion compensation isnot modified, but only the coding of motion parameters, so that theconcept can be integrated in general block-based hybrid video codingschemes with a very small complexity increase. It can also bestraightforwardly extended to more general concepts, in which notrectangular blocks but other sets of samples are associated for uniquemotion parameters. The concept is applicable to multiview coding withadditional depth maps. The disparity information for calculating themotion parameters can be derived based on coded depth maps.

One advantage of the concept presented now compared to conventionaltechniques for employing the motion data of already coded views is thatthe motion/disparity predictors are completely derived based on codedmotion and disparity/depth information, without assuming any particularstructure of the disparity field. At no point, it is not assumed thatthe disparity field can be well approximated by constant translationalor affine parameters for an image; instead actually coded disparityinformation are used for accessing the motion of an already coded view.Further, it is not assumed that the disparity of a macroblock is similarto the disparity of neighboring blocks which assumption is unsecure. Byusing actually coded depth/disparity information, the concept providessuitable disparity estimates for blocks at object boundaries. Further,since no assumption that the motion of the current block is similar tothat of neighboring blocks; is made, improved motion parameterpredictors at object boundaries are provided. Furthermore, the conceptdoes not require any transmission of disparity corrections. further, theconcept does not require modifying the actual motion/disparitycompensation process of hybrid video coding designs for being built intosame. Only the derivation of motion parameters and/or motion parameterpredictors is changed, so that it can be included in conventional videocoding designs without any big modification and has a small complexity.In addition it should be noted that the concept is applicable to thecoding with and without depth maps. Depth maps need not to be coded aspart of the bitstream. Rather, coded disparity vectors may be used forderiving disparities.

The concept described hereinafter can be decomposed into the followingsteps:

-   -   Derivation of depth/disparity data for the current picture of        the current view.    -   Derivation of candidate motion or disparity data for a current        block based on the derived depth/disparity data.    -   Coding of the motion or disparity data for a current block.

In the following, these steps including advantageous embodiments aredescribed in more detail. All steps are described for block-based motioncompensation with translational motion vectors. The concept is, however,also applicable to more general schemes in which a generalized set ofsamples (e.g., a non-rectangular part of a block, or any other shape) isassociated with a unique set of motion parameters; and it is alsoapplicable for coding schemes in which the motion compensation iscarried out using higher order motion models (e.g., affine motionmodels, or other N-parameter motion models).

Before describing the details of the concept, we briefly describe theadvantage and underlying thoughts also valid for the above, more genericembodiment. The basic relationship between the projection of areal-world object point in different views and at different timeinstances in illustrated in FIG. 3. Assuming we know the real motioninside the views and the real disparities between views, thecorresponding motion and disparity vectors are given as follows:

-   -   the motion vector for the current view is given by the        difference of the locations of the projected object point in the        reference picture of the current view and the current picture of        the current view, MV_(C)(x_(C,t))=x_(C,t-1)−x_(C,t)    -   the motion vector for the reference view is given by the        difference of the locations of the projected object point in the        reference picture of the reference view and the current picture        of the reference view, MV_(R) (x_(R,z))=x_(R,t-1)−x_(R,t)    -   the disparity vector for the current time instant is given by        the difference of the locations of the projected object point in        the current picture of the reference view and the current        picture of the current view, DV_(t)(x_(C,t))=x_(R,t)−x_(C,t)    -   the disparity vector for the reference time instant 1s given by        the difference of the locations of the projected object point in        the reference picture of the reference view and the reference        picture of the current view,        DV_(t-1)(x_(C,t-1))=x_(R,t-1)−x_(C,t-1)

Hence, we have the following relationship between the motion anddisparity vectors:

MV _(C)(x _(C,t))+DV _(t-1)(x _(C,t-1))−MV _(R)(x _(R,t))−DV _(t)(x_(C,t))=0

If three of the motion and disparity vectors are given, the fourthvector can be calculated by a simple addition. In particular, thetemporal motion vector for the current view can be derived according to

MV _(C)(x _(C,t))+MV _(R)(x _(R,t))+DV _(t)(x _(C,t))−DV _(t-1)(x_(C,t-1))=0

if the motion vector of the same object point in the reference pictureas well as the disparity vectors at both time instants are given. Inmost cases, the motion vector (or motion parameters) for the referenceview are given, because this view is already coded usingmotion-compensated prediction. But the disparities are usually notgiven, they can only be estimated. However, by using two estimatedvalues the accuracy of the final results may be quite inaccurate and notsuitable for deriving sufficiently accurate temporal motion vectorpredictors. But in general, it is justified to assume that the depth anobject point (distance of the real-world object point from the camera)is nearly constant between the time instances corresponding to thecurrent and the reference picture (the object motion from or to thecamera between two successive pictures is usually much smaller than thedistance of the object to the camera). Then, also the disparities arenearly constant and the relationship between the motion vectorssimplifies to

MV _(C)(x _(C,t))≈MV _(R)(x _(R,t))=MV _(R)(x _(C,t) +DV _(t)(x _(C,t)))

It should be noted that we still need an estimate for the disparity inthe current access unit (current time instant) in order to predictionthe motion inside the current view based on the motion inside thereference view. However, the accuracy of the disparity estimate is lesscritical, since it is only used for accessing motion data in thereference view. The motion compensation operations as well as the codingof motion data is done based on blocks of multiple samples and,furthermore, the motion of neighboring samples or blocks is often verysimilar. Nonetheless, an accurate estimate of the disparity generallyimproves the prediction of motion parameters. The estimated disparityvector DV_(t)(x_(C,t)) can also be used as a disparity vector fordisparity-compensated prediction (i.e., using the picture of the currentaccess unit in a reference view as reference picture), which canrepresent a special mode of the motion and disparity-based prediction ofmotion parameters.

Derivation of Candidate Motion or Disparity Data

In the following, we describe the basic derivation of motion data for agiven block of the current picture in a particular view (that is not thebackwards compatible base view) such as view 22 in FIG. 1, given motiondata of an already coded reference view or of a set of already codedreference views such as 20 in FIG. 1. For this description, we assumethat an estimate of the depth data for the current picture is given suchas 64 in FIG. 1. Later, we describe how this depth estimate can bederived and how the derived motion data can be used for an efficientcoding of the current view. The depth data 64 for the current pictureare either given by a pixel-wise or a block-wise depth map. If apixel-wise depth map is given, the depth map specifies a depth value foreach sample (or each luminance sample) of the associated picture. If ablock-wise depth map is given, the depth map specifies a depth value foran M×N block of samples (or luminance samples) for the associatedpicture. For example, a depth value for each block of the smallestpossible block size (e.g., 4×4 or 8×8 block) that can be used for motioncompensation could be specified. Conceptually, a depth value d given bya sample of the depth map, specifies a function of the real-world depthz, which is the distance between the associated real-world object point(the projection of the real-world object point is the image sample atthe given position) and the camera:

d=ƒ _(dz)(z)

The depth values are given with a particular precision (furthermore,depth values are often estimated, since the actual depths are usuallynot known). In most cases, depth values are given by integer numbers.Given the depth values and particular camera parameters (such as thefocal length, distance between cameras, minimum and maximum depthvalues, or functions of these parameters), the depth value d can beconverted into a disparity vector v=[v_(x), v_(y)]^(T);

v(x)=ƒ_(vd)(d(x),x),

where ƒ_(vd) specifies the function that maps a depth value d at samplelocation x=[x, y]^(T) to a disparity vector. In a particular importantsetup is the one-dimensional parallel camera configuration, which ischaracterized by the following properties:

-   -   all cameras of the camera array are of the same type and have        the same focal length    -   the optical axes of all cameras are parallel and lie inside the        same plane    -   the scan lines of the image sensors are parallel to the plane        that contains the optical axes

In this case, the vertical component of the disparity vector is zero,v=[v, 0]^(T). Each real-world object point has the same verticallocation in all views. Its horizontal location depends on the depth ofthe object point. The difference between the horizontal locations isgiven by the disparity

v=ƒ _(vd)(d).

In an important case, the relationship between the real-world depth zand the depth values d is given in a way that a linear relationshipbetween the disparity v and the depth value d is obtained

v=m _(vd) ·d+n _(vd),

where m_(vd) and n_(vd) are given by the camera parameters. The depthvalues d are usually given as integer values. And for internalcalculations it is usually also advantageous if the obtained disparityvalues are integer values. For example, the disparity v can be expressedin the same units that is used for the motion/disparity vectors inmotion/disparity-compensated prediction (e.g., half-, quarter, oreighth-sample accuracy). In this case, the integer values for thedisparity can be obtained by the integer equation

v=[(m]* _(vd) ·d+n* _(vd))>>u _(vd),

Where “>>” specifies a bit shift to the right (in two's complementarithmetic), and m*_(vd) and n*_(vd) are scaled (and rounded) versionsof m_(vd) and n_(vd), respectively.Using the described basic relationships between the given depth valuesand the actually disparity, we describe advantageous embodiments forusing motion information that are derived based on already coded motioninformation in one or more reference views and the given estimated depthvalues.

Method 1: Switched Motion/Disparity Vector Prediction

In an advantageous embodiment of the invention, the underlying multiviewvideo coding scheme such as for modules 24, 26, 80 and 82 includes amode, in which the following parameters are transmitted as part of thebitstream 21:

-   -   reference picture index specifying a particular (temporal or        inter-view) reference picture of a given a list of reference        pictures. If the given list of reference pictures consists of a        single element, this index is not transmitted but inferred at        the decoder side. The reference picture include temporal and/or        inter-view reference pictures.    -   a motion/disparity vector predictor index specifying a        motion/disparity vector predictor of a given list of        motion/disparity vector predictor candidates. If the list of        motion/disparity vector predictor candidates consist of a single        element, this index is not transmitted but inferred at the        decoder side. For at least one block of a picture, the list of        motion/disparity vector predictor candidates includes a        motion/disparity vector that is derived based on given        depth/disparity information and motion information in an already        coded view.

In addition, a motion/disparity vector difference specifying thedifference between the motion/disparity vector used formotion/disparity-compensated prediction and the chosen predictor(indicated by the transmitted index into the motion/disparity vectorpredictor candidate list) can be transmitted as part of the bitstream.In one embodiment, this motion/disparity vector difference can be codedindependently of the reference index and the chosen predictor. Inanother embodiment of the invention, the motion/disparity vectordifference is coded depending on the transmitted reference index and/orthe chosen predictor. For example, a motion/disparity vector differencecould only be coded if a particular motion/disparity predictor ischosen.

The reference picture list and the motion/disparity vector predictorcandidate list are derived in the same way at encoder and decoder side.In specific configurations, one or more parameters are transmitted inthe bitstream, for specifying how the reference picture lists and/ormotion/disparity vector predictor candidate lists are derived. For theadvantageous embodiment of the invention, for at least one of the blocksof a picture in a dependent view such as 22, the list ofmotion/disparity vector predictor candidates contains a motion ordisparity vector predictor candidate that is derived based on the given(estimated) depth values or based on the given (estimated) depth valueand the motion parameters of an already coded view. Beside themotion/disparity vector predictor that is derived based on the givendepth values and motion parameters of already coded views, the candidatelist of motion/disparity vectors predictors may contain spatiallypredicted motion vectors (for example, the motion/disparity vector of adirectly neighboring block (left or above block), a motion/disparityvector that is derived based on the motion/disparity vectors of directlyneighboring blocks) and/or temporally predicted motion/disparity vectors(for example, a motion/disparity vector that is derived based on themotion/disparity vector of a co-located block in an already codedpicture of the same view). The derivation of the motion/disparity vectorcandidate that is obtained by using the given depth data 64 and thealready coded motion parameters such as 42 of other views such as 20 canbe performed as described in the following.

Derivation Based on the Derivation of a Representing Depth for theCurrent Block

In a first advantageous embodiment of the invention, first arepresenting depth value d for the given block 50 is obtained based onthe given sample-based or block-based depth map. In one advantageousembodiment, a particular sample location x of the given block 50, whichmay be the top-left sample, the bottom-right sample, a middle sample, orany other particular sample, is considered. The depth value d=d(x) thatis associated with the sample (as given by the given block-wise orsample-wise depth maps 64) is used as representing depth value. Inanother advantageous embodiment, two or more sample locations x_(i) ofthe given block (for example, the corner samples or all samples) areconsidered and based on the associated depth values d_(i)=d(x_(i)), arepresenting depth values d is calculated as a function of the depthvalues d_(i). The representing depth value can be obtained by anyfunction of the set of depth values d_(i). Possible functions are theaverage of the depth values d_(i), the median of the depth values d_(i),the minimum of the depth values d_(i), the maximum of the depth valuesd_(i), or any other function. After obtaining the representing depthvalue d for the given block, the motion/disparity vector predictionproceeds as follows:

-   -   If the reference index that is coded for the block 50 refers to        an inter-view reference picture (i.e., a coded picture at the        same time instance as the current picture, but in an already        coded view such as 20), the representing depth value is        converted to a disparity vector v based on given camera or        conversion parameters as described above, v=ƒ_(vd)(d), and the        motion/disparity vector predictor is set equal to this disparity        vector v.    -   Otherwise (the reference index refers to a temporal reference        picture (i.e., an already coded picture of the same view (such        as 22)), the motion vector predictor is derived based on a given        reference view or a set of reference views such as 20. The        reference view or the set of reference views are either        determined by a particular algorithm or a signaled in the        bitstream 14. As an example, the reference view can be the        previously coded view for the same time instant, or it can be        the already coded view (for the same time instant) that has the        smallest distance to the current view, or any other of the        already coded view determined by a particular algorithm. The set        of already coded view can be the set of already coded views for        the current time instant or any subset of this set.

If a single reference view is used, the motion vector predictor isderived as follows. Based on the camera parameters for the current view22 and the reference view 20 or the corresponding conversion parameters,the representing depth d is converted into a disparity vectorv=ƒ_(vd)(d). Then, given the disparity vector v, a sample location x_(r)in the reference view 20 is determined. Therefore, a particular samplelocation x_(r) of the current block 50 is considered, which may be thetop-left sample of the block, the bottom-right sample, a middle sample,or any other sample of the block. The reference sample location x_(r) isobtained by adding the disparity vector v to the particular samplelocation x inside the current block. If the disparity vector v is givenwith sub-sample accuracy, it is rounded to sample accuracy before it isadded to the sample location. Given the reference sample location x_(r),the block 40 (a block is a set of sample that is associated with uniqueprediction parameters) in the picture 32 t ₁ (at the current timeinstant as the current picture) of the reference view 20 that covers thereference sample location x_(r) is determined. If this reference block40 is coded in an inter-coding mode (i.e., a mode that employsmotion-compensated prediction, including the SKIP or MERGE mode), thereference picture or reference pictures that are used for predictingthis block are investigated. Let t_(C,R) be the time instant of thereference picture (in the current view) that is referred to be thereference index that is coded for the current block 50. And let t_(R,R)^(i) be the time instants of the reference picture that are used forpredicting the reference block (which covers the sample location x_(r))in the reference view 20. If one or more of the reference pictures thatare used for predicting the reference block 40 are pictures at the sametime instant as the reference picture given by the reference index forthe current block 50 (i.e., if t_(C,R) is equal to any of the valuest_(R,R) ^(i)), the corresponding motion vectors 42 are used for derivingthe motion vector predictor for the current block 50. If exactly one ofthe time instants t_(R,R) ^(i) is equal to t_(C,R), the motion vectorpredictor for the current block 50 is set equal to the motion vector 42for the reference block 40 that is associated with the correspondingvalue of t_(R,R) ^(i). If two or more of the time instants t_(R,R) ^(i)are equal to t_(C,R), the motion vector predictor is set equal to agiven function of the associated motion vectors for the reference block40. A possible function is to use the first motion vector (in anyparticular order, e.g. by using the first hypotheses with t_(R,R)^(i)=t_(C,R)), another possible function is to use the average of themotion vectors, a further possible function is to use the median of thecandidate motion vectors, or to use the median of the motion vectorcomponents for deriving all components of the motion vector predictor.If none of the associated reference pictures has a time instant t_(R,R)^(i) equal to t_(C,R), the motion vector predictor is marked as notavailable. In an advantageous embodiment of the invention, anon-available motion vector predictor is not included in the list ofmotion/disparity vector predictor candidates. In another advantageousembodiment of the invention, a non-available motion vector predictor isincluded in the list of motion/disparity vector predictor candidates forincreasing the robustness of the parsing algorithm, but it cannot bechosen by an encoder. In another advantageous embodiment of theinvention, a non-available motion vector predictor is replaced byanother defined motion vector, which may be, for example, the zerovector, or a motion vector that is derived using the motion vectors of aneighboring block. Instead of the time instants (t_(R,R) ^(i), t_(C,R)),other parameter that specify a similar measure can be used fordetermining whether a motion parameter set (consisting of a referencepicture index and a motion vector) can be used for deriving the motionvector predictor. For example the picture order count (similarly definedas in H.264) could be used or the reference index could be used.

If a set of two or more reference views are used, the motion vectorpredictor can also be derived based on information in all referenceviews. In an advantageous embodiment, the reference views are ordered ina particular order. As an example, the reference views can be ordered inthe order in which they are coded. Or as another example, the referenceviews are ordered in the order of increasing distances to the currentview. Then, the first reference view of the ordered set is investigatesand the corresponding motion vector predictor is derived. If this motionvector predictor is marked as available (i.e., it is not marked as notavailable), the motion vector predictor is used. Otherwise, if themotion vector predictor is marked as not available, the next referenceview in the given ordered set is investigated, etc. In anotheradvantageous embodiment of the invention, all reference views of thegiven set are investigated and the candidate set of motion vectorsconsists of all corresponding motion vectors that are associated with areference picture for which the associated time instant t_(R,R) ^(i) isequal t_(C,R). The final motion vector predictor is then derived by afunction of the set of candidate motion vectors. A possible function isto use the first motion vector (in any particular order), anotherpossible function is to use the average of the motion vectors, a furtherpossible function is to use the median of the candidate motion vectors,or to use the median of the motion vector components for deriving allcomponents of the motion vector predictor.

For further illustration, the basic process for deriving a motion vectorfor the current block 50 c given the motion in a reference view 20 and adepth map estimate for the current picture 32 t ₂(T) (using a particularsample position inside the current block 50 c) is depicted in FIG. 4using similar reference signs as in FIG. 1 in order to ease the mappingof the description of FIG. 4 onto FIG. 1 so as to serve as a possiblesource of more detailed explanation of possible implementations. Given asample location x in the current block 50 c and a depth value d for thissample location (which is given by the estimate 64 of the depth map), adisparity vector 102 is derived, and based on this disparity vector 102,a reference sample location x_(R) in the reference view 20 is derived.Then, the motion parameters 42R of the block 40 _(R) in the referenceview picture 32 t ₁(T) that covers the reference sample location x_(R)are used as a candidate for the motion parameters for the current block50 c in the current view 22. Or alternatively, a subset of the motionparameters of the reference block is used for the current block 50 c. Ifthe reference index for the current block SOT is given, only motionparameters 42R of the reference block 40 _(R) that refer to the sametime instant T (or picture order count or reference index) as the givenreference index for the current block 50 c or considered.

Derivation Based on Multiple Depth Values for the Given Block

In a second advantageous embodiment of the invention, the current block50 c is not represented by a representing depth, but different depthvalues for different sample locations inside the block are derived andused for deriving a set of candidate motion vector predictors. Given thecurrent block a set of sample locations x^(i) are considered. The set ofsample locations can include the top-left sample, the top-right sample,the bottom-right sample, the bottom-left sample, or a middle sample ofthe block. For each of the sample locations x^(i), a depth value d^(i)is assigned by the given depth map. Depending on whether the givenreference index refers to an temporal or inter-view reference, thefollowing applies.

-   -   If the reference index that is coded for the block 50 c refers        to an inter-view reference picture (i.e., a coded picture at the        same time instance as the current picture, but in an already        coded view), the depth values d^(i) are converted to a disparity        vectors v^(i) based on given camera or conversion parameters as        described above, v^(i)=ƒ_(vd)(d^(i)). Then, the motion/disparity        vector predictor is derived as a function of these disparity        vectors v^(i). The motion/disparity vector predictor can be set        equal to the disparity vectors v^(i) that occurs most often, or        it can be set to the median (or component-wise median) of the        disparity vectors v^(i), or it can be set to the average of the        disparity vectors v^(i), or it can be determined by any other        function of the disparity vectors v′.    -   Otherwise (the reference index refers to a temporal reference        picture (i.e., an already coded picture of the same view)), the        motion vector predictor is derived based on a given reference        view or a set of reference views. For each sample location        x^(i), a depth value d^(i) is derived and mapped to a disparity        vector v^(i). Then, for each disparity vector v^(i) (marked as        available), a motion vector m^(i) is derived by any of the        algorithms specified above (for the first advantageous        embodiment). Then, the final motion vector predictor is given by        a function of the motion vectors m^(i). The motion vector        predictor can be set equal to the motion vector m^(i) that        occurs most often, or it can be set to the median (or        component-wise median) of the motion vectors m^(i), or it can be        set to the average of the motion vectors m^(i), or it can be        determined by any other function of the motion vectors m^(i).        Method 2: Mode for which all Associated Motion Parameters are        Derived

In another advantageous embodiment of the invention, the multiview videocoding scheme includes a coding mode, in which all motion parameters(including the number of hypotheses, the reference indices, and themotion vectors) are derived based on a given depth map 64 and the motionparameters 42 of an already coded view 20. In a particular embodiment ofthe invention, this mode can be coded as a candidate of a list ofcandidate motion parameters (as it is used in the merge syntax in thecurrent HEVC working draft). That means, encoder and decoder derive alist of motion parameter candidates for a block in the same way, whereone of the motion parameter candidates are the motion parameters thatare derived based on the motion of an already coded view 20. Then, anindex is coded that signals to the decoder which of these motionparameter candidates is used. In context of the merge syntax, it can beargued that the current block is merged with a “co-located” (in spiritof representing a similar content) block in a reference view. In anotherembodiment, a specific syntax element signals the usage of the newcoding mode. In a slightly modified version, the number of motionhypotheses that are used for generating the prediction signal can beexplicitly signaled inside the bitstream, and only the reference indicesand the associated motion vectors are derived. In another modifiedversion, motion vector differences can be additionally transmitted inorder to refine the derived motion parameters.

Derivation Based on Multiple Potential Reference Indices

In a first advantageous embodiment of the invention, the derivation ofthe motion parameters for the current block 50 c uses any of theconcepts described for method 1 above and considers more than onepotential reference index. In the following, we first describe how areference index for a particular motion hypothesis (and reference list)and the associated motion vector can be derived. As a first step, anordered set of reference indices for a given reference list isdetermined. This can be for example just a single reference index (e.g.,the first index for the reference list or the first index representing atemporal reference picture), or it can consist of the first two indexesof the reference list, or it can consists of all reference indices ofthe reference list, or it can consist of the first reference index thatrepresents a temporal reference picture and the first reference indexthat is not equal to the first reference index that represents atemporal reference picture (i.e., the second temporal reference picturein the list or the first inter-view reference picture). Any otherdefined set of reference indices is possible. Given the ordered set ofreference indices, the first reference index is considered and a motionvector for this reference index is derived by any of the embodimentsdescribed for method 1 above. If the derived motion vector is marked asnot available, the next reference index is considered and thecorresponding motion vector is derived. This process is continued untilan available motion vector is returned or all reference indices of thelist have been tested. If no available motion vector is found, the finalmotion parameters are marked as not available. In one configuration, notavailable motion parameters are not inserted into the candidate list ofmotion parameters. In a second configuration, not available motionparameters are inserted into the candidate list of motion parameters(for parsing robustness), but an encoder is not allowed to choose notavailable motion parameters. In a third configuration, a not availablemotion parameters are replaced by particular motion parameters, whichmay be, for example, a zero reference index and a zero motion vector ora reference index and motion vector that are derived based on the motionparameters in a spatial neighborhood of the current block. If the newcoding mode is signaled by a particular syntax element and the derivedmotion parameters are not available, the corresponding syntax element iseither not transmitted (and the coding mode is not used) or the encoderis not allowed to select the value for the syntax element that specifiesthe usage of the new coding mode or the not available motion parametersare replaced by a particular motion parameters (see above).

If the number of motion hypotheses or the number of used reference listsis explicitly coded, a set of motion parameters consisting of areference index and a motion vector is determined for each motionhypothesis or reference list as specified above.If the number of motion hypotheses or the number of used reference listsis not explicitly coded, the number of motion hypotheses or the employedreference lists are also derived based on the actual coded motionparameter in the reference view(s). Given a maximum number of motionhypotheses or the maximum set of reference lists that can be used, foreach of the motion hypothesis (reference lists) a set of motionparameters is derived as described above. Then, the number of motionhypotheses (set of used reference picture lists) is given by thehypotheses (reference lists) for which the derived motion parameters aremarked as available. As an example, if we have two potential motionhypotheses and for both motion hypotheses a valid set of motionparameters (reference index and motion vector) is derived, the newcoding mode specifies bi-prediction with the derived motion parameters.If, however, only for one of the hypotheses (reference lists) a validset of motion parameters is derived, the new coding mode specifiesuni-directional prediction (one hypothesis) with the set of valid motionparameters. If for none of the motion hypotheses (reference lists) avalid set of motion parameters is derived, the complete set of motionparameters is marked as not available. In this case, the set of motionparameters is either not added to the list of candidate motionparameters, or it is added (for parsing robustness) but not used by anencoder, or it is replaced by a particular defined set of motionparameters (e.g., with one motion hypothesis, a reference index equal to0 and a motion vector equal to 0). It would also be possible to checkanother set of reference indices for one or more of the potential motionhypotheses.

Derivation Based on a Single Representing Depth Value

In a second advantageous embodiment of the invention, first a referenceblock in the reference view is derived and then the motion parameters ofthis block are used as motion parameter candidates for the currentblock. Here, the number of motion hypotheses as well as the referenceindices and motion vectors are copied from the reference block in thereference view. The basic concept for this embodiment is illustrated inFIG. 2 and has been briefly described above. First, a representing depthvalue d, and based on this depth value a disparity vector v, and areference sample location x_(R) are derived by any of the algorithmsdescribed for method 1. Then, the block (also referred as referenceblock) in the reference view that covers the reference sample locationx_(R) is considered. The motion parameters for the current block (or onecandidate for the motion parameters) are set equal to the motionparameters of the derived reference block. It is also possible that themotion parameters (in particular the reference indices and the number ofmotion hypotheses) are modified, for example in the following scenarios:

-   -   If the reference picture list for the reference view is        constructed in a different way than the reference picture list        for the current view (i.e., a particular reference index does        not always refer to the same access unit for both lists), a        reference index for the current block can be modified in a way        that it refers to a picture at the same time instant (or a        picture with the same picture order count) as the corresponding        reference picture in the reference view.    -   If a reference index in the reference view refers to an        inter-view reference picture, the reference index for the        current view can be modified in a way that it also refers to a        selected inter-view reference picture (for example, the same        inter-view reference picture as the current view or the        reference picture that is represented by the reference view). In        this case, also the motion vector has to be replaced with a        disparity vector, which can be obtained by converting the        representing depth d to a disparity vector.    -   If not for all reference pictures that used in the reference        block a corresponding picture (same time instant or picture        order count or reference index) is available in the reference        list for the current block, the motion hypotheses that refer to        reference pictures that are not available for the current blocks        can be considered as not existing.    -   If the reference block is intra coded, the motion parameters can        be replaced with motion parameters for disparity compensated        prediction. For example, the reference index can be set in a way        that it refers to the reference view picture and the motion        vector can be set equal to the disparity vector that is obtained        by converting the representing depth d to a disparity vector. As        an alternative, the motion parameters could be marked as not        available.        Combination with Method 1

In an embodiment, the coding mode described by an embodiment for method1 (coding of reference index, derivation of a motion vector or motionvector predictor) can be supported in addition to a coding modedescribed by an embodiment of method 2 (derivation of all motionparameters including the number of motion hypotheses, reference indices,and motion vectors or motion vector predictors).

Method 3: Mode for which all Associated Motion Parameters as Well as theBlock Partitioning are Derived

In another advantageous embodiment of the invention, the multiview videocoding scheme includes a coding mode, in which different motionparameters for subblocks of the given block are derived based on themotion parameters in an already coded reference view such as 20 and anestimated depth map 64. Or in other words, the multiview video codingscheme includes a coding mode for a block, in which the partitioning ofthe block 50 c into smaller subblocks as well as the motion parametersassociated with the subblocks are derived based on the motion parametersin an already reference view and an estimated depth map.

For this coding mode, a minimum block size is defined, which may beequal to the minimum block size that is supported formotion/disparity-compensated prediction or may be a multiple of theminimum block size that is supported for motion/disparity-compensatedprediction. If the given current block 50 c is smaller or equal to thedefined minimum block size, the current block 50 c is not split intosmaller block for the purpose of motion/disparity compensation and theassociated motion parameters are derived as described for any of theembodiments for method 2 above. If the given current block is largerthan the defined minimum block size, it is split into subblocks thathave a size equal to the defined minimum block size. For each of thesesubblocks, a set of motion parameters is derived using any of theembodiments for method 2 described above.

If the motion parameters for any of the subblocks are marked as notavailable (for example, because the corresponding reference block 40_(R) is coded in an intra-mode or it only uses inter-view prediction),they can be replaced by motion parameters of any of the neighboringsubblocks for which the motion parameters are available. Such analgorithm can operate in a way that neighboring blocks are tested inspecific defined order (which may depend on the location of thesubblocks to be replaced), and the motion parameters of the subblock tobe replaced are set equal to the motion parameters of the first subblockin the specified order that has valid motion parameters.

In a particular embodiment of the invention, the obtained subblocks witha given minimum block size specify the partitioning of the given currentblocks 50 c. In another embodiment of the invention, the obtainedsubblocks can be combined based on the associated motion parameters inorder to form larger blocks that are used formotion/disparity-compensated prediction. The combination of subblockscan proceed in a hierarchical fashion. Therefore, in the first hierarchystage, sets of four neighboring blocks can be considered. If the numberof motion hypotheses and the associated reference pictures and motionvectors are the same for all four subblocks, the four subblocks aresummarized to a larger block (with motion parameters that are identicalto the motion parameters of the original subblocks). In the nexthierarchy stage, four blocks of the next hierarchy level (consisting of4 original subblocks) are considered. If all four blocks have beensummarized to larger blocks in the previous hierarchy stage and thenumber of motion hypotheses and the associated reference pictures andmotion vectors are the same for all four blocks, these four blocks areagain summarized to a larger block (with motion parameters that areidentical to the motion parameters of the original subblocks). Thisalgorithm in continued up to the highest possible hierarchy level forthe given current block. In the extreme case (if the motion parametersof all subblocks are the same), the entire current block is not splitbut associated with a unique set of motion parameters. In a slightlymodified version, 4 blocks are also summarized to a larger block if themotion vectors are not be exactly the same, but the difference (whichmay be defined as maximum component difference or the absolute value ofthe vector difference) between the motion vectors is less or equal to adefined threshold (the number of motion hypotheses and the employedreference pictures is still the same). In this case, the motion vectorsthat are associated with the larger block are determined as a functionof the motion parameters of the 4 subblocks. Possible functions are theaverage of the motion vectors, the median (or component-wise median) ofthe motion vectors, the motion vector of any particular subblock, or themotion vector that occurs most often in the four subblocks).

In an embodiment of the invention, the coding mode described by anembodiment for method 1 (coding of reference index, derivation of amotion vector or motion vector predictor) can be supported in additionto a coding mode described by an embodiment of method 3 (derivation ofthe blocks splitting as well as all motion parameters including thenumber of motion hypotheses, reference indices, and motion vectors ormotion vector predictors). In addition, a coding mode according to anyembodiment of method 2 may be supported.

Coding of Motion and Disparity Data

As already described above, the usage of coding modes representingembodiments of the invention, needs to be signaled to the decoder. Thiscan be realized in different ways. In one version, a particular syntaxelement (which may be a flag) can be inserted into the syntax, whichsignals whether a conventionally derived motion vector predictor (ormotion vector or motion parameter set) is used or whether a motionvector predictor (or motion vector or motion parameter set) that hasbeen derived using a depth map estimate and motion parameters of analready coded view is used. In another version, the motion vectorpredictor (or motion vector or motion parameter set) that has beenderived using a depth map estimate and motion parameters of an alreadycoded view can be inserted into a candidate list of conventionallyderived motion vector predictors (or motion vectors or motion parametersets) and an index is transmitted which signals which motion vectorpredictor (or motion vector or motion parameter set) is used. Theparticular syntax element or the index into a candidate list can betransmitted using fixed-length coding, variable-length coding,arithmetic coding (including context-adaptive binary arithmetic coding),or PIPE coding. If context-adaptive coding is used, the context can bederived based on the parameters (for example, the particular syntaxelement or the index into a candidate list) of neighboring blocks.

In an advantageous embodiment of the invention, the multiview videocoding scheme includes a coding mode for which one or more motionhypotheses are signaled by transmitting a reference picture index, amotion vector predictor index, and a motion vector difference for eachmotion hypothesis. For this coding mode, a list of candidate motionvector predictors is derived based on the transmitted reference pictureindex and the transmitted index signals which one of the motion vectorcandidates is used. By using the embodiment, one of the motion vectorcandidates (for at least one block) is derived based on a depth mapestimate and motion parameters of an already coded view (see method 1above). In a slightly modified version, a motion vector difference isnot transmitted but inferred to be equal to 0 (either for all motionvector candidates or only for the motion vector candidate that has beenderived based on a depth map estimate and motion parameters of analready coded view.

In another advantageous embodiment of the invention, the multiview videocoding scheme includes a coding mode for which one or more motionhypotheses are signaled by transmitting motion parameter index (or mergeindex). For this coding mode, a list of candidate sets of motionparameters (including the number of motion hypotheses, the referenceindices, and motion vectors) is derived. By using the embodiment, one ofthe candidate sets of motion parameters (for at least one block) isderived based on a depth map estimate and motion parameters of analready coded view (see methods 2 and 3 above). In a particular versionof this embodiment, the candidate set of motion parameters (for at leastone block) that is derived based on a depth map estimate and motionparameters of an already coded view includes partitioning informationfor the current block (see method 3 above). In a slightly modifiedversion of this embodiment, motion vector differences can additionallybe transmitted (potentially depending on the selected set of motionparameters).

In another embodiment of the invention, the multiview video codingscheme includes a coding mode corresponding to method 2 or 3 and thesyntax includes a flag which specified whether this coding mode is used.

Derivation of Depth Map Estimates for the Current Picture

The derivation of motion parameters for a block of the current picture50 c based on the motion parameters of already coded views such as 20,as described so far, involves that an estimate 64 of the depth map forthe current picture is available. As mentioned above, this depth mapestimate 64 can specify a sample-wise depth map (a depth value isspecified for each sample of the current picture) or a block-wise depthmap (a depth value is specified for blocks of samples). The depth mapestimate 64 may be derived based on already coded parameters, such asdepth maps or disparity vectors and motion parameters. In principle, thepossibilities for deriving a depth map estimate 64 for the currentpicture can be categorized into two classes. For one class, the depthmap estimate is derived based on actually coded depth maps. Thecorresponding possibilities described below involve that the coded depthmaps are present in the bitstream (before they are used). Concepts ofthe second class do not require that depth maps are coded as part of thebitstream. Instead, the depth map estimate is derived based on codeddisparity vectors. The second class of procedures can be appliedindependently of whether depth maps are coded as part of a bitstream.This was the case discussed above with respect to FIGS. 1 and 2 forwhich the following description in so far provides individuallytransferrable details regarding individual aspects. It should also benoted that, when depth maps are coded, both classes of methods can beapplied. It is also possible to select different methods for differentframes.

In the following, the basic concept and advantageous embodiments forderiving depth maps estimates (with and without coded depth maps) aredescribed.

Class 1: Derivation Based on Coded Depth Maps

If the depth map that is associated with the current picture 32 t ₂(T)would be coded before the current picture, the reconstructed depth mapcould directly be used as an estimate of the real depth map for thecurrent picture. It is also possible to pre-process the coded depth map(e.g., by applying a filtering it) and use the result of thepre-filtering as the estimate of the depth map that is used for derivingmotion parameters.

In most configurations, the depth map that is associated with aparticular picture is coded after the picture 32 t ₂(T) (often directlyafter the associated picture). Such a configuration allows that codingparameters (such as motion parameters) that are transmitted for codingthe conventional video pictures can be used for predicting the codingparameters that are used for coding the depth maps, which improves theoverall coding efficiency. But in such a configuration, the depth mapthat is associated with a picture cannot be used as an estimate for thedepth map in deriving the motion parameters 54. However, the depth mapfor an already coded view (of the same access unit) such as 20 isusually available and can be used for deriving an estimate of the depthmap of the current picture. At least, the depth map of the base view(independent view) 20 is available before coding any dependent view 22.Since the depth map of any view represents the geometry of the projectedvideo scene to some extent (in combination with camera parameters suchas focal length and the distance between cameras) it can be mapped toanother view. Consequently, if the depth map for the current picture 32t ₂(T) is not available, the coded depth map for an already coded viewof the same access unit 20 is mapped to the current view and the resultof this mapping is used as depth map estimate.

In the following we describe a particular algorithm for realizing thismapping. As described above, each depth value d corresponds to adisplacement vector v between two given views. Given transmitted cameraor conversion parameters, a depth value d can be converted to adisplacement vector my the mapping v=ƒ_(vd)(d). Hence, given a depthvalue d at a particular sample location x_(R) in the reference depth map(already coded), the sample location x_(C) of the same depth value inthe current depth map is obtained by adding the disparity vector tox_(R), x_(C)=x_(R)+v. Hence, each depth value of the reference depth mapcan be mapped to a sample location of the current depth map in order toobtain a depth map estimate 64 for the current picture. However, sinceparts of objects that are visible in one view are not visible in anotherview, there are sample location in the depth map for the current view 22to which more than one depth values is assigned and there are samplelocation in the depth map for the current view to which no depth valuesis assigned. These sampled location may be processed as follows:

-   -   If more than one depth value is assigned to a particular sample        location, it means that a foreground object is displaced in        front of a background object. Consequently, the depth value d        (of the potential depth values) that represents the smallest        distance to the camera is assigned to such a sample location.    -   If more no depth value is assigned to a particular sample        location, it means that a foreground object has moved and the        previously covered background is visible. The best that can be        done for such regions is to assume that the disoccluded        background has the same depth than the neighboring background        samples. Hence, regions to which no depth value has been        assigned are filled with the depth value of the surrounding        samples that represents the largest distance to the camera.

This algorithm is specified in more detail in the following. Forsimplifying the following description, we assumed that larger depthvalues represent smaller distances to the camera than smaller depthvalues (but the algorithm can easily be modified for the oppositeassumption):

-   -   1. All samples of the depth map (estimate) for the current        picture are set to an undefined depth value (e.g., −1).    -   2. For each sample location x_(R), of the reference depth map,        the following applies:        -   a. The depth value d at the sample location x_(R) is            converted to a disparity vector v using the given camera or            conversion parameters, the disparity vector v is rounded to            sample accuracy (if applicable), and the sample location            inside the current picture is derived by            x_(C)=x_(R)+v=x_(R)+[round(ƒ]_(vd)(d)).        -   b. If the depth value at sample location x_(C) in the            current picture has an undefined value, the depth value at            sample location is set equal to the depth value d.        -   c. Otherwise, if the depth value at sample location x_(C) in            the current picture has a defined value d_(x) with d_(x)<d,            the depth value at sample location is modified and set equal            to the depth value d.    -   3. The regions in the current depth map that have undefined        depth values are filled by a particular hole filling algorithm.        For such a hole filling algorithm, the depth value of the        background that is uncovered in the current view is derived        based on the samples of the surrounding depth values. As an        example, the smallest depth map value of the surrounding samples        can be assigned. But more sophisticated hole filling algorithms        are possible.

The algorithm for mapping a depth map of a given view to a differentview is further illustrated in FIG. 5 on the basis of a very simpleexample. FIG. 5 illustrates a possible process for mapping a depth mapsuch as 32 t ₁(T) given for one view 20 to another view 22. At the lefthand side, the given depth map for the reference view is shown, wherethe shaded area represents a background and the white area represents aforeground object; in the middle of FIG. 5, middle, the converted depthmap obtained by displacing the samples of the given map with thedisparity vectors that correspond to the depth values and keeping theforeground object for locations to which more than one sample isprojected, is shown. The black area represents on disoccluded area towhich no sample has been projected. FIG. 5, right, shows the converteddepth map after filling the disoccluded areas by the depth value for thebackground, i.e. ba background filling.

In a particular embodiment of the invention, the hole filling canrealized by a particularly simple algorithm which processes the lines ofthe converted depth map separately. For each line segment that consistsof successive undefined depth values, the two surrounding values areconsidered, and all depth samples of the line segment are replaced withthe smaller of these two depth values (background depth). If the linesegment has only one surrounding depth value (because it is located atthe image border), the depth samples of the line segment are replacedwith this value. If complete lines have undefined values after thisprocess, the same process is applied for the columns of the depth map.

Although the algorithm above has been described for sample-wise depthmaps, it can also be applied to block-wise depth maps (resulting in alower complexity) or the given sample-wise depth map for the referenceview can first be converted into a block-wise depth maps (bydownsampling) and then the algorithm can be applied for the block-wisedepth map.

Class 2: Derivation Based on Coded Disparity and Motion Vectors

If no depth maps are coded as part of the bitstream, an estimate for thedepth map can be generated by using the coded motion and disparityvectors. A basic idea of the following concept can be summarized asfollows. The decoding of a (multiview) video sequence generally startswith a random access unit. The picture for the base view in a randomaccess unit is intra coded without referencing any other picture. Thepictures for dependent views in the random access unit can be intracoded or predicted using disparity-compensated prediction. Typically,most blocks will be coded by disparity-compensated prediction, since itusually gives better prediction results than intra prediction. Since,the coded disparity vectors (which are used for disparity-compensatedprediction) can be converted into depth values (using the inversefunction ƒ_(vd) ⁻¹), the disparity vectors can be directly used forgenerating a block-based depth map that is associated with a dependentview in a random access unit (the depth for intra-coded blocks can beestimated based on the depth for surrounding disparity-compensatedblock). Then, this obtained depth map can be mapped to the base view.The next picture for the base view is typically coded using mostlymotion-compensated prediction. Here, it can be assumed that the motionof the depth data is the same as the motion for the texture information(a depth and an associated texture sample belong to the same objectpoint). Given this assumption, the estimated depth data for the firstpicture in the base view can be motion-compensated for obtaining anestimate for the depth map of the base view in the current access unit.And then, this (motion-compensated) depth map estimate for the base viewcan be mapped to a dependent view for obtaining a depth map estimate forthe current picture (in the current view). If more than two views arecoded, the creation of depth map estimates for the third view, fourthview, ect. can be simplified, since we also have a depth map estimatefor the first two views of the access unit. One of these depth mapestimates (advantageously the base view) can be mapped to the third,fourth, or any following view in order to generate a depth map estimatefor this view.

The idea of generating a depth map estimate is further illustrated bysome figures (showing the processing steps for multiview coding with twoviews as they are performed by depth estimator 28). The coding/decodingstarts with a random access unit, for which the base view picture 32 t₁(0) is intra-coded and the non-base-view pictures 32 t ₂(0) are codedusing only intra and inter-view prediction (but no motion-compensatedprediction). After coding the second view 22 in the random access unit“0”, a block-based depth map estimate for this second view 22 isgenerated 120 using the coded disparity vectors 122 for this view 22, asillustrated in FIG. 6. This depth map estimate 64 ₂(0) for the secondview 22 is then mapped 124 to the first view (base view) 20 and a depthmap estimate 64 ₁(0) for the first view 20 is obtained. It should benoted that for the second view 22 of a random access unit, thederivation of motion/disparity parameters based on the motion parametersof the base view and a disparity estimate cannot be used, because noestimate of the depth map is available when the second view 22 of arandom access unit is coded.

If a third view is coded, the depth map estimate of any of the first twoviews (advantageously the second view) can be mapped to the third viewresulting in a depth map estimate for the third view, which can be usedfor deriving motion parameters for the third view. And after coding thethird view, a block-based depth map can be generated using the codeddisparity vectors for the third view (which can than later be used forgenerating a depth map estimate for any following view). For anyfollowing view, basically the same process as for the third view can beused.

The pictures of the base view in non-random-access units are typicallymainly coded by motion-compensated prediction, since motion-compensatedprediction usually gives better coding efficiency than intra coding.After a picture of the base view is coded, an estimate of the depth mapfor this picture is generated 140 (cp. 71 in FIG. 1) using the motionparameters 42(1) for the picture 32 t ₁(1), as illustrated in FIG. 7.Therefore, each block of the new depth map estimate 64 ₁(1) is created140 by motion-compensating the depth map estimate 64 ₁(0) (cp. 74 inFIG. 1) for the corresponding reference picture or pictures. Thereference pictures and corresponding motion vectors 42(1) that are usedare the reference pictures and motion vectors that are coded in the datastream for the associated picture. The depth samples for intra-codedblocks can be obtained by spatial prediction. This depth map estimatefor the base view is than mapped 142 (cp. 76 in FIG. 1) into thecoordinate system for the second view in order to obtain a depth mapestimate 64 ₂(1) for the second view which can be used for derivingmotion parameters, i.e. to perform inter-view redundancy reduction.

For any further coded view, a depth map estimate can be generated bymapping the depth map estimate for any already coded view (base view,second view, etc.) to the corresponding view.

After actually coding the picture of the second view (or any followingview), the associated depth map estimate can be updated 160 (cp. 77 inFIG. 1) using the actually coded motion and disparity vectors, asillustrated in FIG. 8. For blocks that are coded using disparitycompensation, the depth map samples can be obtained by converting 162the coded disparity vectors 60 to depth values as described above. Forblocks that are coded using a motion-compensated mode, the depth samplescan be obtained by motion compensating the depth map estimate for thereference frame 32 t ₂(0). Or alternatively, a depth correction value,which is added to the current depth map estimate 64 ₂(1), can be derivedbased on the coded motion parameters 42(1) and 54(1) for the current andfor the reference view. The depth samples of intra coded blocks can bepredicted using spatial prediction or using the motion parameters ofneighboring blocks. After an updated depth map estimate 74 for thesecond view has been generated, this depth map estimate 74 is mapped 164(cp. 78 in FIG. 1) to the base view 20 for obtaining a depth map update64′₁(1) (cp. 74 in FIG. 1) for the base view 20.

If more than two views are coded, the depth map update process for theseviews is the same as for the second view. However, the base view depthmap is only updated after the second view has been coded.

The motion compensation operations for depth maps can be eitherperformed using the coded sub-sample accurate motion vectors. It is,however, often advantageous (from a complexity as well as codingefficiency point of view), if the motion compensation operations fordepth maps are performed with sample (or even block) accuracy.Therefore, the actually coded motion vectors are rounded to sample orblock accuracy and these rounded vectors are used for performing themotion compensation. Furthermore, the described concept can be appliedfor sample-wise as well as block-wise depth map estimates. The advantageof using block-based depth maps is a lower complexity and memoryrequirement for all processing steps. With block-based depth maps, eachdepth sample represents the depth for a block of samples of theassociated picture (e.g., 4×4 blocks or 8×8 blocks). All describedoperations can be performed for block-based depth maps in astraightforward way (i.e., by simply considering a lower resolution ofthe depth maps—one depth sample just represents multiple instead of onetexture sample).

Besides the mapping of a given depth map from one view to another (whichhas been described above), the algorithm contains the following basicsteps:

-   -   Creating a depth map based on disparity vectors for a picture of        a random access unit.    -   Temporal prediction of the base view depth map using the motion        parameters of the associated picture.    -   Update of a depth map estimate using actually coded motion and        disparity vectors for the associated picture.

Particular embodiments for these algorithmic steps are described in thefollowing.

Creation of a Depth Map for a Picture in a Random Access Unit

In a particular embodiment of the invention, the creation of a depth mapfor a picture of a dependent view in a random access unit proceeds asfollows. In general, such a picture contains blocks that are coded usingdisparity-compensated prediction as well as blocks that are intra coded.First, all blocks that are coded using disparity-compensated predictionare considered. The disparity vectors are converted into depth valuesand these depth values are assigned to the corresponding samples of thedepth map. If two or more motion hypotheses are used, either onehypothesis is selected or the final depth value is set equal to afunction of the depth values for the individual motion hypotheses (forexample, the average, the median, the maximum, or the minimum). Afterassigning the depth values for all disparity-compensated blocks, thedepth values for intra coded blocks are obtained by spatial intraprediction. In one version, these samples can be obtained by using thesame intra prediction modes that are used for the associated texturepicture. In another version, the depth of an intra-block can be setequal to a depth values that is obtained by a weighted average of thesurrounding samples (or blocks), where the weighting factors can bedetermined based on the used intra prediction modes. In a furtherversion, the depth for an intra-block can be obtained by setting thedepth samples equal to a value that is given by a particular function ofthe surrounding intra samples (e.g., the average, the median, themaximum, the minimum). Other spatial prediction algorithms are alsopossible. The depth assignment for intra-coded blocks can also be doneinside a single loop over the blocks in an image. That means, the blocksare processed in a particular order (e.g., the coding order), and forboth disparity-compensated and intra blocks, the depth values aregenerated in this order (i.e., the depth assignment for intra-codedblocks doesn't need to wait until all disparity-compensated blocks areprocessed).

Temporal Prediction of the Base View Depth Map

In general, pictures of the base view contain motion-compensated blocksand intra coded blocks. The depth values for motion-compensated blocksare derived by motion-compensated prediction of the depth map estimatefor the corresponding reference picture. If a block of the texturepicture is coded using a single motion hypothesis, the depth samples forthis block can be obtained by displacing the depth samples of the depthmap estimate for the reference picture (given by the signaled referenceindex) by the transmitted (or inferred) motion vector. This motioncompensation operation can be performed with the accuracy of thetransmitted motion vectors (which is usually a sub-sample accuracy) orwith sample- or block-accurate motion vectors. If the motioncompensation is performed with sub-sample accuracy, an interpolationfilter is applied for generating the samples at sub-sample positions. Ifthe motion compensation is performed with sample or block accuracy, thetransmitted motion vectors are rounded before they are used. If theblock of the associated picture is coded with more than two motionhypothesis, one of the hypotheses can be selected for motioncompensation of the depth maps, or all motion hypotheses are used bygenerating the final depth map estimate for the block as a weighted sumof the depth prediction signals for the individual motion hypotheses.Furthermore, the depth samples for a block of a given sizes can be setequal to a representing depth. This representing depth can be obtainedby selecting a particular location inside the block and deriving thedepth value for this location using motion compensation, or it can beobtained by defining a function of the motion-compensated depth valuesfor this block. Such a function can be the average of the depth samples,or the median of the depth samples, or the minimum or maximum of thedepth samples, or the depth sample value that occurs most often in theblock.

Update of a Depth Map Using the Coded Motion and Disparity Parameters

As mentioned above, the depth map estimate for a picture can be updatedafter coding the picture. In the following, we describe some embodimentsof such an update algorithm. Blocks of the picture are processed in aparticular order (for example, the coding order) and for each block, thefollowing applies:

-   -   If the block has been intra-coded (in the associated texture        picture), the depth sample values for this block can be derived        by spatial prediction using the samples of neighboring block.        Some examples for such a spatial prediction technique have been        described above. The depth sample values can also be obtained by        motion compensation using the motion parameters of a neighboring        block (see the description for motion-compensated blocks below).        It is sometime advantageous if the intra block are processed        after all motion- and disparity-compensated blocks are        processed, because then more neighboring are available and can        be used for spatial prediction of depth samples or motion        parameters.    -   Otherwise, if the block 1s coded using one or more disparity        vectors (disparity hypotheses), the depth samples are derived by        converting the disparity vectors to depth values. If only one        disparity vector (disparity hypothesis) is used for the block,        the depth value if given by the corresponding disparity vector.        If two or more disparity vectors are used for the block, one of        the disparity vectors can be selected for deriving the depth        value, or for each disparity vector a depth value can be derived        and the finally assigned depth value is obtained by applying a        function of the individually derived depth values. Possible        functions are, among others, the minimum or maximum of the        individual depth values, the median of the depth values, the        average of the depth values, or the depth values that occurs        most often.    -   Note that a block that is coded using a disparity vector may        additionally be associated with a temporal motion vector. In        this case, the temporal motion vector can be ignored. Or the        derived depth values can be combined with depth values that are        derived for temporal motion hypotheses (see below) in any        specific way (e.g., by averaging these two signals).    -   Otherwise, the block is coded using only temporal motion        hypotheses and the temporal motion hypotheses are used for        updating the depth samples for the block.

In a first embodiment of the invention, the depth map samples arederived by straightforward motion compensated prediction using the depthmap estimates associated with the reference pictures for the currentview. This motion compensation operation can be realized by any of theembodiments for temporal prediction of the base view depth map describedabove.

In a second embodiment of the invention, the depth map samples are notsimply motion compensated, but instead a depth correction value isderived based on the motion vector coded for the current view and themotion vector coded for the corresponding block in the reference viewand this depth correction value is added to the depth map estimated inorder to obtain an updated depth map estimate. The advantage of such anapproach is that depth changes between two instances can be considered.

Let d_(prd) be the current depth estimate for a sample or a block, letm_(curr) be the motion vector that is actually used formotion-compensation of the current block, and m_(ref) be the motionvector for the reference block in the reference view (the referenceblock is derived by using a disparity vector given by the depth estimated_(prd) as described above). Furthermore, let v_(t-1) be the disparityvector in the reference access unit and let v_(t) a be the disparityvector in the current access unit. Given the basic equation for theinterrelationship between the motion and disparity vectors that wederived in the beginning of the description,

m _(curr) +v _(t-1) −m _(ref) −v _(t)=0,

the current disparity vector can be set equal to

v _(t) =v _(t-1)+(m _(curr) −m _(ref)),

if we assume that the coded motion parameters and the disparity in thereference picture are correct. The disparity vector can be convertedinto a depth and vice versa. Hence, the current depth can be expressedas

d=d _(t-1)+ƒ_(vd) ⁻¹(m _(curr) −m _(ref)),

The depth d_(t-1) is the depth value given in the reference image, itcan be accessed using the motion vector for the current block. It isalso possible to replace the depth in the reference image with thecurrent estimate for the depth d_(prd), since this depth has beenderived using motion compensation from the depth in a reference image.

We showed the basic concept for obtaining an updated depth value usingthe motion vectors of the current and the reference block. This basicconcept can be used in a variety of actual implementations. For example,if more than one motion vector is used for the current block and/or thereference block, a single motion vector can be selected or differentdepth values can be derived for the different motion vector and thefinal depth is obtained by using the average (or any other function) ofthe individually derived depth values. Furthermore, only those motionhypotheses in the current and the reference block should be consideredthat use the same reference access unit (or reference time instant, orreference picture order count, or reference index). If such motionhypotheses don't exist, the conventional motion compensation processdescribed above can be used or the depth sample can be marked as notavailable and are later replaced by suitable post-processing steps.Furthermore, the new depth value can be derived based on a single samplefor a block, or for all samples of a block, or for a selected subset ofthe samples of a block. When different depth values for differentsamples are derived, separate depth values can be assigned for differentregions of the current block (e.g., for all 4×4 blocks inside thecurrent block), or the different depth values are used for deriving arepresenting depth for the entire block, e.g., by using the average, themedian, the minimum, the maximum value, or the depth values that occursmost often. The depth map update process can also be combined with somepre-processing steps. For example, not available depth values can bereplaced by the depth values of neighboring samples or blocks.

Regarding the afore-outlined description of methods 1-3, it should benoted that these different alternatives of polishing up the motion ormotion/disparity vector/parameter candidate list by adding respectiveinformation gained from a block, such as block 40 _(R) in the referenceview, would also be advantageous if used with other types of determiningthe disparity vector used to identify the respective block 40 _(R) ofthe reference view. In accordance with this embodiment, the depthestimator 28 in the decoder of FIG. 1 would be optional, just as thedepth estimator 84 in the encoder of FIG. 2 would be.

In particular, in accordance with the latter aspect, the abovedescription of methods 1-3 also revealed an apparatus for reconstructinga multi-view signal into a multi-view data stream, the apparatuscomprising a dependent-view reconstructor 26, which is configured to dothe following in order to reconstruct the dependent-view 22. Referenceis made to the schematic illustrations of the multi-view signal of FIGS.1 and 4, in order to describe the functionality. In particular, thedependent-view reconstructor 26 derives, for a current block 50 c of acurrent picture 32 t ₂(T) and the dependent view 22, a list of motionvector predictor candidates by firstly determining a disparity vector102 for current block 50 c representing a disparity between the currentpicture 32 t ₂(T) of the dependent view 22 and the current picture 32 t₁(T) of the reference view 20 of the multi-view signal at block 50 c. Inorder to do so, the dependent-view reconstructor 26 uses motion anddisparity vectors associated with a previously decoded portion of themulti-view signal such as motion/disparity vectors of pictures 32 t ₁(T)and 32 t ₂(T−1). In the other embodiments outlined above, the estimateddepth-map associated with the current picture 32 t ₁(T) of the referenceview was used as a basis for determining the disparity vector 102, withthe estimation and updating of the depth map estimate using the motionand disparity vectors of previously coded pictures of the dependent viewas well as the reference view having been described above, and in thisregard the above description shall be incorporated for the currentembodiment as well, but in principle, other possibilities exist as well.For example, the dependent-view reconstructor 26 couldspatially/temporally predict a disparity vector for the current block 50c and could use this predicted disparity vector as the disparity vector102.

Then, the dependent-view reconstructor 26 determines block 40 _(R)within the current picture of the reference view using the determineddisparity vector 102, and adds a motion vector to the list of motionvector predictor candidates, which depends on the motion vectorassociated with the determined block 40 _(R), i.e. motion vector 42R.

As described above, in deriving the list of motion vector predictorcandidates, the dependent-view reconstructor could also be configured tospatially and/or temporally predict one or more further motion vectorsfrom spatially and/or temporally neighboring blocks of the dependentview 22, i.e. blocks spatially and/or temporally neighboring currentblocks 50 c. The one or more further motion vectors or a version derivedtherefrom would then be added to the list of motion vector predictorcandidates by the dependent-view reconstructor 26.

The dependent-view reconstructor extracts, for block 50 c, indexinformation specifying one of the list of motion vector predictorcandidates from the multi-view data stream and, in order to be morespecific, from the dependent-view portion 22 thereof. As it is assumedthat the current block 50 c is subject to motion-compensated prediction,i.e. is associated with a temporal prediction mode, the dependent-viewreconstructor 26 reconstructs block 50 c by performing amotion-compensated prediction of block 50 c using a motion vector whichis equal to, or at least depends on, the specified motion vectorcandidate, i.e. the one indexed or specified by the index information.The overhead associated with the enlarged list of motion predictorcandidates is comparatively low compared to the gain in motion vectorprediction quality resulting from the adding of the motion vectorcandidate 42R determined from the reference view.

As has also been described above, the motion information extracted bythe dependent-view reconstructor 26 for the current block 50 c need notbe restricted to the index information. Rather, the dependent-viewreconstructor 26 could further be configured to extract, for block 50 c,a motion vector difference in relation to the specified motion vectorcandidate and to perform the reconstruction of block 50 c such that theused motion vector further depends on a sum of the motion vectordifference and the specified motion vector candidate, i.e. the onespecified by the index information out of the list of motion vectorpredictor candidates.

In the above, motion- and disparity-compensated prediction have beenstrictly distinguished. However, the difference between both may vanishif, for example, the same mode is used to signal both, with thedifference between both merely being derivable from an index indexingthe picture relative to which the motion/compensated prediction is to beperformed. Then, the just-described functionality of the decoderdependent-view reconstructor could be rewritten by replacing “motion”with “motion/disparity” as there would be no difference. Naturally, themeasures should be taken so that the vector candidate actually specifiedrefers to the same type of previously coded picture, i.e. temporallypreceding or in view direction preceding, or that the even the adding tothe list is restricted conditionally performed accordingly.

According to the above-described method 1, the dependent-viewreconstructor 26 is configured to extract for block 50 c, further areference picture index specifying a reference picture of a list ofreference pictures, comprising the current picture 32 t ₁(T) of thereference view 20 and the already decoded pictures 32 t ₂(t<T) of thedependent view 22, and the dependent-view reconstructor 26 may beconfigured to, with the reference pictures as one of the already decodedpictures of the dependent view 22, perform the motion-compensatedprediction using the one already decoded picture of the dependent viewas specified by the reference picture index as a reference, and if thereference picture is the current picture 32 t ₁(T) of the referenceview, add the determined disparity vector 102 or a modified disparityvector derived from the determined disparity vector 102 to a list ofdisparity vector prediction candidates, extract index informationspecifying one of the list of disparity vector predictor candidates fromthe multi-view data stream and reconstruct block 50 c by performing adisparity-compensated prediction of block 50 c using a disparity vectorwhich depends on the specified disparity vector candidate using thecurrent picture 32 t ₁(T) of the reference view 20 as a reference.Again, the difference between motion-compensated anddisparity-compensated prediction could be dissolved. The same predictionmode could be signaled for block 50 c. As to whether motion-compensatedor disparity-compensated prediction is actually performed by thedependent-view reconstructor 26, would be defined by the referencepicture index which indexes into a buffer or list of reference picturescontaining both, temporal predecessors, i.e. previously decoded picturesof the dependent view, as well as view predecessors, i.e. previouslydecoded pictures of other views.

As became clear from method 2, the dependent-view reconstructor 26 couldalso be configured to perform the derivation of the list of motionvector predictor candidates via a list of motion/disparity vectorpredictor candidates being a list of motion/disparity parametercandidates each including a number of hypotheses and, per hypothesis, amotion/disparity motion vector and a reference index specifying areference picture out of such a common list of reference picturesjust-outlined. The dependent-view reconstructor may then be configuredto add motion/disparity parameters to the list of motion/disparityparameter candidates which depend on motion/disparity parametersassociated with a determined block 40 _(R), and to reconstruct block 50c by performing motion/disparity-compensated prediction on block 50 cusing motion/disparity parameters which depend on a motion/disparityparameter candidates specified by the index information. The motionparameters could, as described above, concurrently determine the numberof hypotheses, and a reference index, and a motion/disparity vectordifference per hypothesis. As has also been described above, the numberof hypotheses could be determined beforehand such as by way of the typeof the picture.

And as described in method 3, the dependent-view reconstructor mayadditionally be configured to additionally adopt the partitioning forblock 50 c from block 50 _(R) as far as the motion/disparity predictionis concerned.

The encoder of FIG. 2 would, in accordance with the latter aspect, beconfigured to act accordingly in encoding the multi-view signal into themulti-view data stream. In particular, the dependent-view reconstructor26 would derive, for block 50 c, a list of motion vector predictorcandidates in the same manner. That is, a disparity vector for block 50c representing a disparity between the current picture of the dependentview 22 and the current picture of the reference view 20 of themulti-view signal at the current picture of the dependent view 22 wouldbe determined via motion and disparity vectors associated with apreviously encoded portion of the multi-view signal. Then, a block 50_(R) within the current picture of the reference view would bedetermined using the determined disparity vector, and a motion vectorwould be added to the list of motion vector predictor candidates, whichdepends on a motion vector associated with the determined block of thepicture of the reference view. The dependent view reconstructor wouldinsert, for block 50 c, index information specifying one of the list ofmotion vector predictor candidates, into the multi-view data stream, andencode the block 50 c by performing a motion-compensated prediction ofthe block 50 c using a motion vector which depends on the specifiedmotion vector candidate.

Although some aspects have been described in the context of anapparatus, it is clear that these aspects also represent a descriptionof the corresponding method, where a block or device corresponds to amethod step or a feature of a method step. Analogously, aspectsdescribed in the context of a method step also represent a descriptionof a corresponding block or item or feature of a correspondingapparatus. Some or all of the method steps may be executed by (or using)a hardware apparatus, like for example, a microprocessor, a programmablecomputer or an electronic circuit. In some embodiments, some one or moreof the most important method steps may be executed by such an apparatus.

Depending on certain implementation requirements, embodiments of theinvention can be implemented in hardware or in software. Theimplementation can be performed using a digital storage medium, forexample a floppy disk, a DVD, a Blu-Ray, a CD, a ROM, a PROM, an EPROM,an EEPROM or a FLASH memory, having electronically readable controlsignals stored thereon, which cooperate (or are capable of cooperating)with a programmable computer system such that the respective method isperformed. Therefore, the digital storage medium may be computerreadable.

Some embodiments according to the invention comprise a data carrierhaving electronically readable control signals, which are capable ofcooperating with a programmable computer system, such that one of themethods described herein is performed.

Generally, embodiments of the present invention can be implemented as acomputer program product with a program code, the program code beingoperative for performing one of the methods when the computer programproduct runs on a computer. The program code may for example be storedon a machine readable carrier.

Other embodiments comprise the computer program for performing one ofthe methods described herein, stored on a machine readable carrier.

In other words, an embodiment of the inventive method is, therefore, acomputer program having a program code for performing one of the methodsdescribed herein, when the computer program runs on a computer.

A further embodiment of the inventive methods is, therefore, a datacarrier (or a digital storage medium, or a computer-readable medium)comprising, recorded thereon, the computer program for performing one ofthe methods described herein. The data carrier, the digital storagemedium or the recorded medium are typically tangible and/ornon-transitionary.

A further embodiment of the inventive method is, therefore, a datastream or a sequence of signals representing the computer program forperforming one of the methods described herein. The data stream or thesequence of signals may for example be configured to be transferred viaa data communication connection, for example via the Internet.

A further embodiment comprises a processing means, for example acomputer, or a programmable logic device, configured to or adapted toperform one of the methods described herein.

A further embodiment comprises a computer having installed thereon thecomputer program for performing one of the methods described herein.

A further embodiment according to the invention comprises an apparatusor a system configured to transfer (for example, electronically oroptically) a computer program for performing one of the methodsdescribed herein to a receiver. The receiver may, for example, be acomputer, a mobile device, a memory device or the like. The apparatus orsystem may, for example, comprise a file server for transferring thecomputer program to the receiver.

In some embodiments, a programmable logic device (for example a fieldprogrammable gate array) may be used to perform some or all of thefunctionalities of the methods described herein. In some embodiments, afield programmable gate array may cooperate with a microprocessor inorder to perform one of the methods described herein. Generally, themethods are advantageously performed by any hardware apparatus.

While this invention has been described in terms of several embodiments,there are alterations, permutations, and equivalents which fall withinthe scope of this invention. It should also be noted that there are manyalternative ways of implementing the methods and compositions of thepresent invention. It is therefore intended that the following appendedclaims be interpreted as including all such alterations, permutationsand equivalents as fall within the true spirit and scope of the presentinvention.

REFERENCES

-   [1] ITU-T and ISO/IEC JTC 1, “Advanced video coding for generic    audiovisual services,” ITU-T Recommendation H.264 and ISO/IEC    14496-10 (MPEG-4 AVC), 2010.-   [2] A. Vetro, T. Wiegand, G. J. Sullivan, “Overview of the Stereo    and Multiview Video Coding Extension of the H.264/MPEG-4 AVC    Standard”, Proceedings of IEEE, vol. 99, no. 4, pp. 626-642, April    2011.-   [3] H. Schwarz, D. Marpe, T. Wiegand, “Overview of the Scalable    Video Coding Extension of the H.264/AVC Standard”, IEEE Transactions    on Circuits and Systems for Video Technology, vol. 17, no. 9, pp.    1103-1120, September 2007.

1. An apparatus for reconstructing a multi-view signal coded in a datastream, comprising: a dependent view reconstructor configured for:processing a list of plurality of motion vector candidates associatedwith a coding block of a current picture in a dependent view of themulti-view signal, the processing including comprising: estimating afirst motion vector based on a second motion vector associated with areference block in a current picture of a reference view of themulti-view signal, the reference block corresponding to the coding blockof the current picture in the dependent view, adding the first motionvector into the list of plurality of motion vector candidates, andextracting, from the data stream, index information that specifies atleast one of the list of motion vector candidates to be used formotion-compensated prediction; and reconstructing the coding block inthe current picture of the dependent view by performing themotion-compensated prediction based on the at least one of the list ofmotion vector candidates.
 2. The apparatus of claim 1, wherein thedependent view reconstructor is configured for obtaining a disparityvector with respect to the coding block of the current picture in thedependent view, wherein the disparity vector represents a disparitybetween the current picture of the dependent view and the currentpicture of the reference view, and identifying the reference block inthe current picture of the reference view based on the disparity vector.3. The apparatus of claim 2, further comprising a depth estimatorconfigured for: obtaining motion data associated with the currentpicture of the reference view; applying the motion data associated withthe current picture of the reference view to a depth map estimate of aprevious picture of the reference view to generate a first estimateddepth map for the current picture of the reference view; deriving asecond estimated depth map for the current picture of the dependent viewbased on the estimated depth map for the current picture of thereference view; and determining the disparity vector based on the secondestimated depth map for the current picture of the dependent view. 4.The apparatus of claim 3, wherein the disparity vector for the codingblock in the dependent view is determined further based on at least onedisparity vector associated with one or more previously decoded codingblocks of the multi-view signal.
 5. The apparatus of claim 1, whereinthe dependent-view reconstructor is further configured for: extracting,from the data stream, a motion vector residual directed to the selectedmotion vector; obtaining a refined motion vector based on the selectedmotion vector and the motion vector residual; and performing themotion-compensated prediction to predict the coding block based on therefined motion vector.
 6. The apparatus of claim 1, wherein the list ofmotion vector candidates include one or more of: a first estimatedmotion vector determined based on motion data associated with a spatialneighbor coding block in the current picture of the dependent view, asecond estimated motion vector determined based on motion dataassociated with a temporal neighbor coding block in a previous pictureof the dependent view, a first modified estimated motion vector derivedby modifying the first estimated motion vector, and a second modifiedestimated motion vector derived by modifying the second estimated motionvector.
 7. A non-transitory machine-readable medium having informationstored thereon for reconstructing a multi-view signal coded in a datastream, wherein the information, when read by the machine, cause themachine to perform a plurality of operations comprising: processing alist of plurality of motion vector candidates associated with a codingblock of a current picture in a dependent view of the multi-view signal,comprising: estimating a first motion vector based on a second motionvector associated with a reference block in a current picture of areference view of the multi-view signal, the reference blockcorresponding to the coding block of the current picture in thedependent view, adding the first motion vector into the list ofplurality of motion vector candidates, and extracting, from the datastream, index information that specifies at least one of the list ofmotion vector candidates to be used for motion-compensated prediction;and reconstructing the coding block in the current picture of thedependent view by performing the motion-compensated prediction based onthe at least one of the list of motion vector candidates.
 8. Thenon-transitory machine-readable medium of claim 7, the plurality ofoperations further comprising: obtaining a disparity vector with respectto the coding block of the current picture in the dependent view,wherein the disparity vector represents a disparity between the currentpicture of the dependent view and the current picture of the referenceview; and identifying the reference block in the current picture of thereference view based on the disparity vector.
 9. The non-transitorymachine-readable medium of claim 8, wherein the obtaining the disparityvector comprises: obtaining motion data associated with the currentpicture of the reference view; applying the motion data associated withthe current picture of the reference view to a depth map estimate of aprevious picture of the reference view to generate a first estimateddepth map for the current picture of the reference view; deriving asecond estimated depth map for the current picture of the dependent viewbased on the estimated depth map for the current picture of thereference view; and determining the disparity vector based on the secondestimated depth map for the current picture of the dependent view. 10.The non-transitory machine-readable medium of claim 9, wherein thedisparity vector for the coding block in the dependent view isdetermined further based on at least one disparity vector associatedwith one or more previously decoded coding blocks of the multi-viewsignal.
 11. The non-transitory machine-readable medium of claim 7, theplurality of operations further comprising: extracting, from the datastream, a motion vector residual directed to the selected motion vector;obtaining a refined motion vector based on the selected motion vectorand the motion vector residual; and performing the motion-compensatedprediction to predict the coding block based on the refined motionvector.
 12. The non-transitory machine-readable medium of claim 7,wherein the list of motion vector candidates include one or more of: afirst estimated motion vector determined based on motion data associatedwith a spatial neighbor coding block in the current picture of thedependent view, a second estimated motion vector determined based onmotion data associated with a temporal neighbor coding block in aprevious picture of the dependent view, a first modified estimatedmotion vector derived by modifying the first estimated motion vector,and a second modified estimated motion vector derived by modifying thesecond estimated motion vector.
 13. An apparatus for encoding amulti-view signal coded into a data stream, comprising: a dependent viewencoder configured for processing a list of plurality of motion vectorcandidates associated with a coding block of a current picture in adependent view of the multi-view signal, comprising: estimating a firstmotion vector based on a second motion vector associated with areference block in a current picture of a reference view of themulti-view signal, the reference block corresponding to the coding blockof the current picture in the dependent view, adding the first motionvector into the list of plurality of motion vector candidates,determining at least one of the list of motion vector candidates to beused for reconstructing the coding block in the current picture of thedependent view via motion-compensated prediction, and generating indexinformation specifying the at least one of the list of motion vectorcandidates; and inserting, into the data stream, the index information.14. The apparatus of claim 13, wherein the dependent view encoder isconfigured for obtaining a disparity vector with respect to the codingblock of the current picture in the dependent view, wherein thedisparity vector represents a disparity between the current picture ofthe dependent view and the current picture of the reference view, andidentifying the reference block in the current picture of the referenceview based on the disparity vector.
 15. The apparatus of claim 14,further comprising a depth estimator configured for estimating thedisparity vector by: obtaining motion data associated with the currentpicture of the reference view; applying the motion data associated withthe current picture of the reference view to a depth map estimate of aprevious picture of the reference view to generate a first estimateddepth map for the current picture of the reference view; deriving asecond estimated depth map for the current picture of the dependent viewbased on the estimated depth map for the current picture of thereference view; and determining the disparity vector based on the secondestimated depth map for the current picture of the dependent view. 16.The apparatus of claim 15, wherein the disparity vector for the codingblock in the dependent view is determined further based on at least onedisparity vector associated with one or more previously decoded codingblocks of the multi-view signal.
 17. The apparatus of claim 13, whereinthe dependent-view encoder is further configured for: extracting, fromthe data stream, a motion vector residual directed to the selectedmotion vector; obtaining a refined motion vector based on the selectedmotion vector and the motion vector residual; and performing themotion-compensated prediction to predict the coding block based on therefined motion vector.
 18. The apparatus of claim 13, wherein the listof motion vector candidates include one or more of: a first estimatedmotion vector determined based on motion data associated with a spatialneighbor coding block in the current picture of the dependent view, asecond estimated motion vector determined based on motion dataassociated with a temporal neighbor coding block in a previous pictureof the dependent view, a first modified estimated motion vector derivedby modifying the first estimated motion vector, and a second modifiedestimated motion vector derived by modifying the second estimated motionvector.